Heat makes the wafer expand, that, at some point pushes on the button on the snap switch shutting off the heat. As the wafer cools it contracts lifting off the snap switch, turning on the heater.
The idea is to get it to shut off as close to 99.5 as possible & to come back on at 99.5. Most wafers aren't that accurate, so you probably end up with it shutting off at 100 & coming back on at 99. If you get one that is that accurate, you've got a good one, but remember its ADVERAGE temperature you want. So if it comes on at 99 & off at 100 the average is 99.5 - perfect.
One last note - once you get it set, make sure you lock the set screw, normally with a wing nut, to keep the adjustment from moving. You may need to lock it down & loosen it, & reset several times to get the perfect temperature, because when you lock it, it can change the distance between the wafer & snap switch, changing the temperature.. Kinda of a pain, but once its set, it's set