if you want to stop by skidding, you still have to unweigh the rear wheel at least a little bit in order to start a skid. once you're skidding you can apply more weight to the rear. you can also hop skid, where you pull the rear wheel off the ground, and lock the pedals in mid-air. when the wheel goes back down it will skid. how much weight you apply to the rear depends on how strong your legs are. you need pretty strong legs to maintain a skid with full weight on the rear. people usually skid several times before they come to a full stop.