Foiling upwind at 30 knots indication of how much speed boat can be pushed to.
Most of the attention yesterday may have been Emirates Team New Zealand's speed record of 47.2 knots - but the really significant news involved the Kiwis going 30 knots upwind.
In about 18 knots of breeze. You don't even need to be a sailor to know how much that defies the laws of physics in these amazing AC72 yachts.
It is now clear that upwind will be a key battleground for the America's Cup. In the seventh race of the Louis Vuitton final - which ended in a 6-1 lead in the first-to-seven series - Team NZ produced another masterclass. They won the start and, with another on-demand burst of acceleration in the sprint to the short first mark, relegated Luna Rossa to a second place they never relinquished.
However, in about 18 knots of wind, gusting to 21 or so, the Italians were a lot more competitive downwind. In both downwind legs, they kept the deficit down to 23s and 6s. Luna Rossa also touched new highs - hitting 46.8 knots themselves.
But upwind, they were literally blown away. The Kiwis have been working hard on their upwind speed and touched on 30 knots for a short period; their average upwind speed would have been somewhere between 20-24 knots. The Italians couldn't live with it. They fell behind by 1m 25s on the upwind leg to form the major part of a 1m 58s loss at the finish line. It was another indication of how the America's Cup teams are developing the speed of these boats, day by day, week by week.
"There's no plateau here; they are still discovering more speed - the simile is that it is like breaking the sound barrier," said John Bertrand, the Australian skipper of Australia II who won the cup in 1983. "They will go beyond 50 knots though we will have to see how relevant that is round the race track."
That's the point. Top-end downwind speed in these remarkable yachts can be paid for but the price can be stability. There is only one upwind on the five-leg course for the cup. But it takes only about 30 minutes to sail the course and the upwind leg takes more than 30 per cent of that time. So upwind speed shapes as a major element.
Oracle and Team NZ have been working hard on it and there is no clear idea who is faster yet.
The Kiwis are employing mostly a semi-foiling style - lifting Aotearoa out of the water only enough to have the hulls lightly skimming the waves. Oracle have only been seen foiling higher - which can bring windage and leeway problems - but who knows what they have managed to develop in training.
Foiling upwind brings major speed advantages but also carries a price. Sometimes, to maintain the speed, the boat has to be steered off course. It's all about sailing close to the wind and finding the most direct route to the mark while maintaining extra speed.
Oracle coach and tactician Darren Bundock said yesterday: "One of our strengths is sailing upwind. The boat has been designed for that. It has a good aerodynamic package.
"There's a lot of talk about foiling upwind and both teams are foiling sometimes. Sometimes it's best to do it and sometime's it isn't ... "
Team NZ skipper Dean Barker agreed: "The boat is not perfectly set up for foiling upwind; there can be control issues. It's a real balance between angle and speed."
Luna Rossa helmsman Chris Draper, however, mentioned upwind speed as Team NZ's biggest improvement in this regatta.
After yesterday's race Draper spoke again about the Italian team's frustration in improving vastly - only to find the Kiwis improving as well.
"Upwind they are just phenomenal," he said.
"Thirty knots is just unbelievable. I think it would be fun to see how these boats would go [upwind] in another cycle, say two or three years from now."
If you needed proof of the advances that have been made by both teams, remember that, in the initial round robin races, the Italians trailed in by five minutes or more. It emerged they had covered more than 1km extra of ground around the course.
Yesterday, in race 7, the distance covered stats were almost identical - Team NZ covered 11.4nm; Luna Rossa 11.5nm.
So that is one of the major fascinations of this America's Cup - foiling upwind and the mixture of science, technology, human judgment and geometry it brings to bear.
* Race 8-8 10am
* Race 9 (if required)-9 10am.