I’ve done this Valsalva Maneuver many times particularly while flying. It’s best to be very gentle while doing it, i.e., not blowing forcefully.
But one time I did this while descending after a long overseas flight, and it caused a serious problem for me. I was already in great pain from pressure buildup inside the ear before I started trying to do it. None of the swallowing or jaw-movement tricks that often help (and should always be tried first) gave any relief. It probably didn’t help that I’d been up in the Alps at elevations ranging from 8,000 - 14,000 feet for a week before coming down to Zurich and then flying home. Anyway, I don’t know what I did wrong, but the already-bad pain in one ear increased greatly when I pinched my nose and blew gently, and I had a strong sensation of the airplane spinning out of control for about 30 seconds even though it was flying A-OK. Good thing I wasn’t the pilot at the time! It took several days to feel better. Clearly I’d caused a serious problem for my middle ear doing this.
After that disquieting experience, I started taking pediatric nasal decongestant spray along with me on the plane. People advised spraying just before take-off to prevent pressure build-up, and mostly, this worked very well. Now I’m on high blood pressure medicine, and all those sprays are forbidden to me, because they contain epinephrine derivatives that temporarily raise blood pressure. Also, decongestant spray users should know that many doctors say using those sprays for more than a day or two consecutively will cause serious and very unpleasant rebound-congestive symptoms. I now think most people should not use those sprays, and I no longer do.
But I’m very careful now before trying a Valsalva Maneuver. The combined sensations of dizzy spinning and increased pain were not something I ever want to repeat.