We may never know exactly why, at 12.55am on August 8, 2014, inside his Gold Coast apartment, Gable Tostee pressed the voice record button on his smartphone.
But it will be a drunken decision that either clears him or convicts him.
No explanation has ever been proffered to the six men and six women of the Brisbane Supreme Court jury who will this week decide his fate as to exactly why Tostee chose to secretly start recording inside his 14th floor apartment, where he and his Tinder date, New Zealand tourist Warriena Wright, had come four hours earlier.
But had he not done so, Tostee's defence barrister Saul Holt, QC, said in his closing address to jurors on Friday, his murder trial would have played out very differently.
"Thank goodness he did ... thank goodness he did press record on his app at 12.55," Holt said.
"Can you imagine for a moment this case if that recording had not existed?
"Would anybody, anybody, have believed without bursting out in unmitigated laughter an account in which Ms Wright throws rocks at Mr Tostee?
"Hitting him with a metal clamp? The things she said and did over the course of the evening? Any explanation of that kind would have been laughed out of court.
"This case would just have involved what the neighbours heard, that messed up scene inside the house and the fact that she's dead, on the floor, 14 storeys below.
"The recording is the key to this case.
"The Crown called 22 witnesses before you and still the only thing that really matters is the recording."
Within little more than an hour of Tostee pressing record, Wright had plunged from his balcony to her death on the driveway below, allegedly while trying to flee from him.
Neighbours have testified they were woken by the 26-year-old's terrified screams for help, so concerned by her alarm they went to investigate, only to witness the horror of her fatal fall.
But it is the extraordinary, and almost unprecedented, recording that has been most revealing of what occurred in that apartment that night, and which will be the key piece of evidence on which jurors will decide if Tostee caused his Tinder date's death.
Thanks to the recording, both Crown prosecutor Glenn Cash and Mr Holt agree on almost all crucial facts of the case.
They agree Wright was not pushed or thrown by Tostee from the balcony, but that he forced her out there and closed the sliding door.
They agree that he was not out there when she fell, and that she lost her grip and fatally plunged while attempting to climb to the balcony below.
They also agree that early in the evening, Wright was the aggressor in an ongoing series of fights the pair can be heard having, and that his blood on two ornamental rocks found inside the apartment supports his claim she was throwing them at him.
But it is the six crucial minutes before he shuts her out on the balcony that Cash has argued constitute murder.
Her repeated screams of, "no, no, no, no" and "just let me go home", Cash argued, can only be interpreted as a young woman in sheer terror.
A woman in fear for her life.
For the first time on Friday, however, via Holt's closing address, jurors heard Tostee's version of what happened inside his apartment that night.
In those six minutes, Holt said, Tostee restrains an "increasingly erratic" Warriena Wright, who had been throwing rocks at him, and asks her to leave his apartment, telling her she was "not my kind of girl" and that, if she tried anything, he would "knock you the f*** out."
When he apparently let her up to leave, Holt said, Wright armed herself with the metal clamp of a telescope and began hitting him in the head with it.
Tostee again physically restrained her, moving her onto the balcony he had earlier threatened to throw her off in a bid to "de-escalate" the situation, Holt told the court said.
"He moved her bodily and physically though the nearest open door and he shut the door and locked it," Holt said.
"Within seconds of that happening, Warriena Wright climbed over the 14th storey balcony and fell to her death.
"That sequence of events is a desperate tragedy but it is not murder and it is not manslaughter.
"Somebody has died and our reaction is to want to fix it.
"Just because somebody is dead does not in itself mean someone is criminally responsible for that death."
Holt said Tostee had a legal right to act to remove Ms Wright from his apartment.
"Gable Tostee was lawfully permitted to restrain Ms Wright because she attacked him with rocks," he said.
"He was acting to remove a disorderly person from his property and the law says you can do that.
"Locking her on the balcony, shutting and locking the door, was an act of de-escalation ... to intervene, an act that created safety, in essence, for both of them."
He said Ms Wright remained the aggressor throughout the night, and Tostee was patient, "gentlemanly, even" with her.
"She was not behaving in a sober and reflective and mature way ... she was irrational the entire night," he said.
"She was three times the legal (driving) limit ... she was massively, massively drunk and she was behaving enormously strangely."
Holt submitted Tostee was not the "cartoonish villain portrayed in the media" as the Tinder killer.
"He has told her to leave, he's let her up and he's told her if she tries anything he'll knock her out," he said.
"The reality is, she then tries something. She picks up the metal clamp and hits him.
"She is clearly on the table holding that object. It's clearly the object being referred to when he's saying 'let go, let go, let go'."
"He was hit, probably in the head, with a telescope clamp.
"Those are absolutely critical points. Tostee was plainly acting in self-defence, if hit in the head, or even if an attempt to hit in the head, with a heavy metal object.
"When it drops, that's when she gets hysterical, all of the, 'no, no, no, no' and the screaming happens from that point onwards.
"She hits him in the head and at that point he puts her outside."
But Cash argued Wright's chilling screams, along with her rushed attempt to climb over the balcony, indicated a person in "abject terror".
"What would drive Warriena Wright to attempt such a thing in the early hours of August 8," Cash asked.
"The prosecution says fear. Fear of the defendant, fear of Gable Tostee, fear of what he would do to her if he let her back inside."
He said while Wright may have been the aggressor in the beginning, the tables turned in those six minutes, evidenced by Tostee's menacing tone of voice and refusal to let her go home despite her pleas because, "you've been a bad girl".
Cash said Tostee locking her on the balcony without her phone was the intimidating act that left her with just two options, and which, he argued, made Tostee guilty of murder.
"She couldn't get assistance using the telephone that left her with two options," he said.
"One, to go back in to engage with the man who had violently restrained her ... who had not let her go home.
"In light of what she had done and what she feared he would do, her only reasonable option, the only remaining option, was to climb down the balcony, out of fear for what he would do if she came back in contact with her."
Cash argued that in the six minutes prior to her fall, the noises on the tape made it clear Tostee had restrained her in some way that restricted her breathing, most likely by way of a chokehold, or holding his forearm to her throat.
"The intimidation was undoubtedly not the sole cause of her death but a substantial and significant cause," Cash told jurors.
"Has he unlawfully caused his death?
"You have heard from Warriena Wright, you have heard her progression from drunk and playful to erratic to ... finally screaming in abject terror as she is forced onto the balcony by Gable Tostee, and that speaks more powerfully than I ever could."
Holt told urged jurors to disregard Tostee's calm demeanour after Ms Wright's fall, where he was captured on CCTV footage walking aimlessly around Surfers Paradise and, at one point, buying pizza.
"The media might be desperately interested in whether it was appropriate for Mr Tostee to buy pizza after these events," he said, turning his focus to members of the press sitting in the court.
"Quite how long a person is supposed to wait to eat food after an event presumably none of the media have ever experienced, is beyond me."
"There may be a lot of public comment about the rights and wrongs of a date on Tinder, or the words of the accused when he first communicated with Ms Wright or the morality of the way in which they conducted themselves in the apartment. None of that is relevant."
Also for them not to consider, he urged them, were the possible reasons behind Tostee making the recording, the key piece of evidence on which Holt's case relies.
"The question of whether you think it's okay, or moral or decent or just weird for Gable Tostee to record this evening, is entirely irrelevant," he said.
The jury will return to the courtroom on Monday, where Justice John Byrne will deliver his final address.
They will then retire to consider their verdict.