The Bay then trailed 12-0 in the second half until Mason Emerson used his pace and strength on the left flank to brush off three would-be tacklers to score a try to start a revival at 12-5.
Hayden Hann scored a try with quick flicks from Matt Garland and Emerson for Trent Hape's conversion to hit the front, 14-12, and two minutes left.
Fittingly, Fomai put Manawatu out of their misery with 30 seconds to go after earlier charging down a 22m kick before Jeriah Mua's weighted pass to make it 21-12.
"The boys went back to the basics in the last game and did what they had to do to take the win," Russell said after a day of persistent rain at Playford Park, where the fourth seeds were put into pool A with top seeds Wellington, No 4 Whanganui and No 8 Horowhenua Kapiti in the eight-team tourney following East Coast's withdrawal.
"The Taranaki boys took their opportunities and handled the conditions, which they are probably used to better than we are with the reports of 29 degrees in Hastings," he said, but emphasised captain Neria Fomai and his troops were not going to reach for excuses.
They will view their shortcomings in a video session and endeavour to address them to take their game to a new level.
"We have a couple of weeks before Christmas so we'll take a short break and get back on the horse come the beginning of January."
So was media-shy ex-international Ioasa, who emphasised patience on TV, content?
"Job done. He's never happy so that's all I can say," Russell said with a laugh.
Wellington retained their crown, thumping Taranaki 33-5 in a seven-minute final rather than 10.
Bay women's coach, Daniel Nukunuku, echoed similar sentiments after securing a nationals invite in finishing fourth in their six-team competition.
"We're over the moon with how we've progressed over all the games we've played today," said Nukunuku, rotating players to expose all of them with the nationals in mind.
"We've got areas to work on but, most of all, we have injuries to take care of over four to five weeks to be ready for the next battle."
Captain Julie Ferguson-Ngawaka scored in the quarter-final against Wellington, with Felicity Powdrell converting, but her predominantly young charges couldn't handle the aggression and pace of the losing finalists to succumb 39-7.
In the playoffs, the Bay lost 17-5 to Taranaki despite a try from Nina Pineaha-Hyslop after a bullocking run from Laurae Blake to lead 5-0 within two minutes.
Teilah Ferguson, who got a frantic medical clearance from NZRU 20 minutes before the tourney kicked off, made a great tackle but Naki scored a converted try next and Pineaha-Hyslop had a chance to put them ahead before halftime but lost the ball trying to switchhands for a fend with no one in front of her just past the halfway mark.
Cruelly they camped in the opposition 22 in the second half but fumbles and feeble tackles proved costly.
Manawatu, with Bay's Rhiarna Ferris and Kirsten Duffill, defended their title 26-5 over Wellington.