Well...it lets them know as they approach the boundary (wouldn't be much help if it just let you know when you were outside it). It blinks faster and faster as they near the boundary line and turns solid when they go past it (and they get a penalty). They also have some sort of distance readout because I hear them call the distance to the boundary - but I'm not sure what that device is or where it is located. The race committee also appears to have the ability to reprogram the devices when they reset the course. I suspect that the red blinky on the boat actually gets it's signal from race command - where the GPS signals from the boats are processed in the judges command room (where they monitor the extremely precise GPS data/course map for penalties and reduce the need for on the water judges).

They were also experimenting with anchor-less mark boats that use thrusters and gps units to maintain position. I presume they're using this because I didn't see any anchor rodes on the mark boats (that I noticed). Feed all this back to a central computer station and all the devices get tied together with the same data. Pretty cool.