I feel like you may not have played the games?

You seem to be thinking of the portals as though *they* are moving the cube. Think of it more as a doorway and less as a teleporter. It's a single continuous connection between two points in space. Let's say you're standing and a hula hoop passes over around you. How does your body know how to move as the hoop passes around you? The simple answer: The hoop is not affecting how YOU are moving.

When it comes to portals, it's just that once the hula hoop passes around you gravity may be different. You can place a portal on each side of a flat wall and it would function exactly like a hole... So maybe think of the hula hoop thing again but this time it's a board with a portal on either side. It would function exactly the same as if it were just a hula hoop passing around you.