Correct me if I'm wrong, but I was always under the impression that in the half halt, you incorporated seat, weight, and hand. How I understand it, is you maintain your good position with your hands with a light contact, seat and body supple, and weight centered. For a brief second you transfer your weight slightly back, encouraging lightness of the front end, transferring the horses weight to the hind end. Seat opens up to still encourage forwardness. And the hand closes, going from a softer, supple hand, to a firm one for that brief half a step. To me if your hand remains the same as it was before, you as asking for lengthening, the hand disallows more forward motion, and condenses it to the hind end. I'd love to hear what you have to say about this!
The seat is what coils the energy in the hindlegs. The hand is still neutral to encourage the neck to telescope forwards. Watch Edward Gal in a piaffe - his hands actually physically move forward to allow the horse a freer range of motion with the neck to balance the intense coiling of the hindleg that comes from his seat.
The seat must learn to half halt. It cannot drive all the time. Then it is too harsh and can scare a sensitive horse (think Matthias Rath on Toto). Or on a less sensitive horse will dull the horse to the aids.
Also - in a good connection the contact is not light. It is supple, it is pliable and easy to manipulate, but the horse has quite a bit of pressure on the bit from telescoping the neck into the connection.