Goal: A one-way gate that requires help from the other player to traverse.

The upper player must be the one who pushes the oPushBlock out of the way. The lower player doesn't have anything to grab onto above the lava, to open the door himself.

Also, the 3-block drop functions as a straightforward one-way gate, preventing the "upper" player from entering back into where the lower player came from.

This gate can be turned into an AND gate by making the lower player grab something before this gate, that the upper player needs later.

(for instance, I once put a jetpack down there, and required them to traverse a jetpack-traversable-only section just below)