There seems to be a directional force on things, like a current. Shown most dramatically on Universe's opening, being that such a long trip gave a lot of push. So I'm guessing if you tried to stick your hand back into it, the field would push back.
And it would need that push, because what if something just rolled into a gate gently. Can't have that get stuck, so it's like an automatic clearing effect of the wormhole.
Remember the one Atlantis one where the puddle jumper gets stuck. They have to give enough of a push to get the rest of it moving into the gate, but it came out a bit faster than that push was.