When descending, you've got to blow a little air into your mask to avoid "mask squeeze". Since I tend to breath through my nose a bit anyways (bad habit and tends to make mask leak/fog), I hardly ever have to consciously remember to blow out my nose. As you descend, you'll feel some pressure on your face...just remember to blow a little air into your mask to relieve the pressure. If blowing air into the mask doesn't relieve the pressure, it might be something else, like sinus issues.
When ascending, the excess expanding air has a lot of escape routes. Worst case, if your nose was somehow blocked, the excess air could just bubble out of the sides of your mask. The mask-to-face seal isn't strong enough to keep expanding air inside.
The "reverse block" stuff is usually in reference to ears/sinuses. As you descend, you've got to equalize the air space in your ears and sinuses so it is in equilibrium with the surrounding pressure. The sinuses usually take care of themselves, but to equalize the space in the middle ear, you've got to find some way of opening the Eustachian tubes. There's a lot of ways to do this - you can hold your nose and gently blow, swallow, rotate your jaw forward, etc. If you are unable to equalize while descending, that's called a "block" and if you keep descending, you might wind up with middle ear damage.
As you ascend, the expanding air has got to find a way to come out. Usually, it comes out quite well on it's own (oftentimes, without your even being aware of it). However, if your Eustacian tubes or sinuses become irritated and swollen while at depth (this usually happens from forcing your ears to equalize instead of being gentle - diving with a cold/sinus congestion can cause this, too) then the escaping air won't be able to come out. This is called a "reverse block" and can be quite painful. Unfortunately, while descending is an option, ascending is mandatory!
You can help relieve a reverse block by descending a few feet, wiggling your jaw, etc, in an effort to get those tubes to pop open. Making a really slow ascent seems to help. The good thing, is that reverse blocks don't seem to happen very often. In 130 dives or so, I've only had a reverse block once and that was my own stupid fault. When I felt the pain, I stopped my ascent, spent a min or two getting my ears to equalize and finished my ascent. Unfortunately, I had caused some middle ear damage that took a week or two to heal.
The key is to make your ascents/descents slow; stop when you feel pain, and don't force your ears to clear.
Hope this helps!
