I'm not sure there's a book in this if that's what you're thinking!
The ballerina heel.
I've speculated that TUHU is mostly the product of heel use being associated with hard problems and hard climbing. People think "I'm on a hard problem here, I'd best use my heel as that's what you do on hard problems". Has anybody else observed this?
Back onto Advanced Techniques - I was about to suggest a Ninja Kick as something I'd only seen used on Cypher, but then I remembered having to do one on a 6C+!