If you need to move a snail that is firmly attached, say, to glass, grasp the shell, gently slide the snail along the surface in any direction that is clear, until he releases or reduces his grip. It will let go at some point. Then pick him up. Best not to drop into new tank, instead, place on substrate or rock. Avoids chance of shell damage, even though they will drop fast on their own, I prefer not to drop them.