I believe the way the nose pinch works is that when done while awake, you pinch your nostrils and try to breathe through them for a second, but because it's blocked you won't be able to push the air through. When in a dream, you're still physically breathing in the waking world and that sensation is felt while dreaming, but when you pinch your nostrils there is no physical blockage so even though it feels like your nose is pinched, your breathing is still normal. I've done it a couple times and it has worked very well, it's a very weird sensation to plug your nose but still feel the air passing through it, which is why it makes for a good reality check because that sensation can only happen when dreaming.