When a needle is wedged into a canal and the plunger on the syringe is moved forward, pressure escapes the syringe and builds up in the canal since the canal beyond the end of the needle is simply an extension of the lumen of the needle.
The first thing that happens is that the air that is ahead of the needle gets compressed.
Since the air cannot escape coronally it must move apically.
If there is patency then the air first and then the cement can flow through the apex.
If there is no patency then the air gets compressed and the cement moves apically.
Once the pressure is relieved the air expands and pushes the cement back from the apex.
Thus for pressure filling to work, the apex must be patent.
The downside of pressure filling has always been, that there is no real good way to determine just when the cement gets to the apex.
Thus since we haven't been able to figure this out either, we have stuck with passive filling.