What happens if one would try to go through a incoming wormhole? I just started re-watching the entire series again, and time and time again I think about this.

I remember an episode where O'neill has his arm through the wormhole to keep it open. But he came from the other side, and probably kept is arm in there.... I don't know. I can't remember if this has been answered in the series already.

But what would happen if he took his arm back with him and tried to push it into the event horizon of the incoming wormhole? Would it go through? If so, wouldn't that just defeat the entire purpose of dialing in and out?