Match racing allows "hunting" - the starboard tack boat is allowed to drive the port tack boat low during their duck. That's been legal
in match racing as long as I've followed match racing rules. However, it's only legal
as long at the starboard tack boat does not drive the port tack below 90 degrees to the wind
. Basically, they can't force the give-way boat into negative VMG. It is not legal for fleet racing after the start.
In the race
today it's definitely a bit iffy whether the Kiwi's got away with one or not in my opinion. They actually appeared to sail below 90 true briefly in forcing Oracle down, which is legal, but Oracle had to drop below 90 degrees in response. However, Oracle didn't really give it a good duck at first, and ended up having to go below 90 degrees to complete the duck at least partially because they didn't turn down far enough early on. Their half-hearted duck along with the Kiwi's good play acting at driving back up probably drew the foul.