I am not the best at explaining things but I'll try my best.
Think of your regular contact as a following steady light elastic feeling and try to maintain that feeling. You would half halt with just enough resistance to feel a slight shift in your horse's weight from front to back and then go back to your regular following steady elastic contact, it should only last for a few seconds. First of all be sure that you are riding with a straight line from bit to elbow. Remember to not lock your elbows, think of that line from bit to elbow as an elastic that follows your horse's natural movement of his head and neck. You will notice the movement at walk and canter the most and just be aware to follow it maintaining that elastic feeling. At the trot you will not feel any forward and back movement to follow but you need to be aware of keeping your elbows unlocked so that they open and close with your rising and sitting if you are posting it isn't as noticeable if you are sitting. Ok, so if you want to half halt to shift your horse's weight back you will need to resist the forward movement, starting at your elbows don't pull back, just resist for a few seconds and stay elastic, you may have to add a squeeze at the same time for a few seconds. If you get no response try it with a little bit heavier stronger elastic feeling in your elbow and squeeze a bit stonger at the same time. If your horse does not respond bring them down to halt from walk or walk from trot, etc. It is really difficult to say just how much "pressure" you will need as it varies from horse to horse, what level of training they are at, etc.
Remember, the most important thing is to have your horse moving forwards into the elastic feeling.
Hopefully this was an ok description and it helps you out a little, if not maybe someone else will have a better description for you.