I don't usually use arm pit unless fighting with a beginner because an experienced fighter keeps his elbows down better. You really need to create the opening. I've certainly heard a lot about H1 point though.
There is a condition called "crutch palsy" wherein people with temporary need of crutches, and inadaquate physical therapy to teach them how to use them, lean on the crutches with their armpits instead of supporting their weight on the hand rests and using the tricept muscles more to lift their weight. They get numb and weak in the arms. I'm surprised they don't get problems in the brachial plexus or in the heart. Maybe it is because the armpit pad is flat and soft and doesn't really dig into the point like a punch would.
The older I get, the better I was!