Underneith the crushers, in the floor, is an object which kills the player if they are touching the top/bottom of it. It's there because the crusher pushes Sonic through the floor instead of actually killing him. The floors in Marble Garden Zone are not flat, they ramp up and down. Under some of the crushers the floor is such that it ramps down just before ramping up where the crusher is. If Sonic is moving fast enough, he can be "technically" inside the ground because of the down ramp, and next frame, the collision system will put him on top of the up ramp floor so you don't actually see him inside it. Unfortunately, he can touch the killing object before that happens.