As I get further down the rabbit hole of half-halts and understanding their execution and purpose, a few things stand out.
First of all, as referenced in my last blog post, if the rider is riding with correct biomechanics/power lines and basic contact there is a small half-halt in EVERY stride. A moment of go, followed by the horse stretching to the bit, hitting the rider's connecting power line, slightly pushing off the bit and shifting back slightly on the haunches, the rider softening, and the horse striding forward in go again. Just by the rider being connected properly with the horse there is an almost self perpetuating circle of energy that leads to the horse being more engaged and balanced than it would be with a less correct and connected rider. We've all seen this when a "talented" rider gets on basically any horse and immediately makes it look better without doing anything visible go the naked eye. Lovely to watch and really hard to figure out how to do...
Second off, are the more obvious half-halts that combine many different variations of go and whoa. Sometimes the horse needs much more go and only a little whoa. Other times the whoa is the stronger emphasis, with all sorts of degrees between the two being needed at different times with different horses in different situations. The mechanics of these half-halts are as above in the self perpetuating circle of energy but with more emphasis on either the go or the whoa or both, using either more leg/seat for go or connecting power line/hand for more whoa.
My big take away has been that without being correctly connected it's pretty much impossible to do things well as a rider, but after the seat/biomechanics are good and the horse is going in a correct, balanced, basic frame then there is a whole other world of stronger half-halts that, with having the horse straight, lead to the much more complex work. The circle of energy is the base that the more obvious half- halts build upon.
I think as dressage riders it is so easy to get confused, because we think of the half-halt as being one set of aids given at one strength, but truly it is a range of aids that can be just a whisper or can be as loud as a shout. The best riders seem to do most of their half-halting at a whisper because of their superior connection and biomechanics. Once in a while you will be able to see a really strong use of the aids, but most of the time it is not as necessary because the engagement and balance are already there and the rider just keeps refining the work, using some slightly stronger half-halts to improve the horse, and things get better and better.