I can't help you with how to switch the modes--call the company or a local tri shop for technical assistance. I think you should wear it throughout the race (again, provide you are wearing a wet suit or tri top). Even if you never look at it during a race, it would be useful to know what your heart rate vs. perceived exertion levels are. Knowing that 175 bpm is your sprint race-pace heart rate on the run will help you know what is too high when you do longer races. I wear mine for every workout. If my HR approaches 180 on the run, I know I will be hitting a wall in a couple miles if I don't back off.