Many sailors don't realize that rule 18.3 (Tacking When Approaching a Mark) applies at an upwind finish just like it does at a windward mark. Rule 18.3 is the rule that was added to discourage boats from approaching the windward mark on port tack. But it also limits what port tackers can do when finishing near the pin (port) end of the finish line.
In the situation shown here, the Blue boat (B) tacks on the lee bow of the Green boat (A) so she can beat A to the favored pin end of the line. When B gets to the point, she "shoots" the line and beats A. It's a common way for races to end.
But let's look a little closer here. The two boats were approaching the pin end mark on opposite tacks. and then the Blue boat tacked inside the zone while the Green boat was fetching the mark. Therefore, rule 18.3 applies and puts several limits on the tacking boat (B). Since B caused A to sail above closehauled, B broke rule 18.3.
This is a little-known application of the rule that comes into play fairly often, especially now that the zone around the pin end mark is three lengths!