A half halt is just pressure and release, you squeeze gently and release almost straight away. It's not meant to halt the horse but slow them down.
True half-halts can be used to slow a horse down and steady them, but they have WAY more uses than that. A half-halt is a pre-command, so it lets the horse know something is coming up. It can be used for downward transitions, AND upwards, and it can be used before you signal for about any command.