Exactly as Spyder said. Yes you need a release, in fact a horse learns from the release of pressure, not the pressure itself. So by holding and maintaining the half halt you are continuing the pressure and not teaching the horse anything, as without the release there is no reward gained for coming onto the bit.
However, the release is such that it is more a light give of the hand and release of the seat. Your half halt should be performed mainly through your seat, with the reins acting as a support.
Apply your half halt, using your seat to 'block' the movement for half a stride and a SMALL close of the reins. The second you get a reaction from the hind legs, allow your seat forward quietly, and give your hand very slightly, so you keep a contact with the bit. The mistake that most people make with the release, is to 'throw away' their reins when they get a reaction. Remember that the half halt is intended to create a connection between the hind end and forehand, so if you get that connection, then by throwing the reins away you are breaking it, and you have to expect the horse to come above the bit.