I also believe it's the mainsail/boom .....

...so therefore this is a Port-Starboard crossing currently .... not a windward-leaward crossing ... I do believe .....

You know that a camera's lens can really shorten distances .... when the boom comes onto port the spin boat will no longer be the RoW vessel, but will turn more to port and pass clear ahead of the leaward boat on port ..... It looks like the windward boat is trying to clear their downwind "bear-off/escape" lane ... while the spin boat had starb rights I think they were worried about trying to bear-off in a puff .... even when on Starboard you can not make any radical changes in course that can not be anticipated by a GW/Burdon Vessel .....

Harry