Also don't forget: A half halt isn't just the reins!
I can't tell you how often I hear that when I ask someone what a half halt is... a half halt is a combination of the rein(s), the seat and the legs. Some half halts involve both reins, some involve one. Some half halts you can't even see them being done, others require a lot more force and are clearly visible... it always depends on the situation. :)