RANT - What is a half halt? 

What is a half halt?
This isn't a true rant, but rather a way to share information. Tricky thing with a half halt is that not everyone agrees how to do it, though there are guidelines.
With Collection, pretty much all the dressage masters agree with what it is.
Books I reference.
Horse Gaits, Balance, and Movement by Susan E. Harris
Easy-Gaited Horses by Lee Ziegler
Training Strategies for Dressage Riders 2nd Ed. by Charles De Kunffy
Dressage Formula by Erik F Herbermann
The Complete Training of Horse and Rider by Alois Podhajsky
Dressage in Harmony by Walter Zettl
The Education of Horse and Rider by Jean Froissard and Lily Powell