Competitors are given a fixed zone for racing within, and a fixed period of time. (For example, a lake or a bounded box, with the time being 1pm to 2pm.)
Races usually run for one hour, or half an hour.
At the end of the race, competitors submit their GPS and the software calculates who won the race by travelling the furthest.
Replays allow competitors to dynamically observe the race from above, and see who was winning at various points, and how they were doing.