I feel like a half halt should come before things like transitions and changes in pace. The half halt is like a way of making sure your horse is listening, and more importantly, balancing him, before you ask for a movement. I think a half halt should help to bring your horse under you more (if he isn't already) and should result in him waiting attentively for your next direction. So therefore, I believe the half halt doesn't necessarily create sync... it creates balance.
Every ride, good or bad, teaches you something new.