Here's how I always phase it. Imagine you have a shovel and you are using that shovel to flick some dog shit into your neighbours garden.

With no portal the shit hits the shovel and you flick it, transfering the speed of the shovel into the turds. You stop the shovel and the turds fly away.

Now imagine the shovel has a big rusty hole in it. So it's like a n shape. No portal yet. You go to flick the dog dumps but you just pass straight over them with the hole and the dumps go nowhere. The dumps have gained no momentum because nothing touched them and transferred that to them.

Now put a portal on the end of the shovel. As you sweep it over the cack has anything touched them? Has any object transferred it's momentum to the dog eggs? No, so the dumps just gently tumble out of the other side of the portal.