the way I got to grips with it was to put legs on, grow tall, bear down, squeeze/ hold with the hands for a second before the movement...... when I school I tend to do a lot of talking and my voice commands are..." and prepare to xyz and xzy"
the easiest way i find to remeber the movement is to prepare for the halt, halt then ask straight away or halfway through the halt to go forward....you should then have the hesitation which makes a half halt...
I found I was trying too hard.... so my trainer said to me 'think of it as telling the front end to slow down and wait for the back end to catch-up' and it worked for us - couldn't describe how we do it though!
The perfect half-halt should be almost nothing- a very slight downward action of the seat and the tiniest strengthening of the outside rein. It's desgned just to re-balance the horse and put it more on the hind legs.
Hence a more nvice horse requires a more obvious aid and more help to re-balance and the advanced horse needs a very subtle minor re-adjustment.