Well, in the actual Sonic Game, (I forgot which one it was) it shows Knuckles pulling a switch at one point in the first level. Basically though, he pulls the switch and the ground collapses, and Sonic falls down into the water I think, which is the next level. Me memory isn't the greatest though, but I know he pulls a switch and makes the ground (Well, technically it wasn't ground, it was some kind of metal platform type thing) fall down, and Sonic goes down with it.