What happens is, when you shut your motor down after a hard pull or riding, the water pump quits moving water through the motor. The motor sits and transfers all of it's built up heat to the stagnant coolant that's in the motor. When the coolant gets to hot, it trips the temp sensor. With the bypass switch you interrupt the temp sensors signal to the ECU, making the ECU think everything is good. The sled starts, and the water pump starts, moving the hot coolant in to the heat exchangers and cool water back into the motor. The temp sensor resets with the cool water, you reconnect the sensor to the ECU and you're good to go. In the turbo world, we run the motor harder and hotter and in many cases the coolant passes though the turbo, picking up lots of heat. If you don't let the sled cool down at idle after a hard pull, the motor will heat soak. There's a little more to it than above but that's the cliff notes version.
do a search. The obove was posted by bighoe a while back. You will find tons of info on this by searching. I have a bypass switch but have yet to have to use it. If you let your sled cool down after long pulls, you can actually hear the r's drop, then shut it off you will be fine.
