Slitherine

Under what circumstances can an enemy unit be forced to retreat forward?

Into any open hex?
Into any open hex in the ZOC of one friendly unit?
Into any open hex in the ZOC of two friendly units?

Into any friendly hex when there is no place to retreat backwards.

This is a trick the attacker can use if the defender retreats e. g. behind a river and forms a double defense line there. Then you bombard one hex (often at a bend) and attack this unit without taking control of the hexes vacated by the defender. That means the defender will often retreat across the river to the wrong side. Then it can easily be destroyed and you have a hole behind the river your armor units can exploit.

When done correctly, it's devastating, especially in the opening moves as Germany vs. Poland in 1939.