I have a problem with some of the explanations. The thought is there, though.
It seems simple, lifting your horses forehand while driving the back. However, the cause of heaviness on the forehand is a balance issue. So if you begin retraining by holding up your hands and asking more leg, your horse, who has not been trained to carry himself properly, is going to lean on your hands. As soon as you drop them, your horse could tumble, and you would follow right over his head.
I am training my horse to do the same. I like to work in a large circle, because the inside flexion (poll) is causing her to lighten on my hands. With gentle squeezes on the reins, (like a sponge) I'm asking her to stretch towards the bit. She now has a pretty head set, but only for a step or two at the most. After I have rewarded her with a subtle "give" of my hands (either opening your fingers or pushing your hands forward) I remind her again that I want her to be light, and start the process again. It's important to ask for lightness at the walk, trot, and eventually canter. Transitions really help. My exercise is just what I use for training my horse, and it's going to be different for everyone. The idea is there, asking for lightness, rewarding, and repeating. You may see silght results by the end of your work, but it may be a quicker response to your hands. Real enforcement and work in this area will ultimately be the "cure" for a heavy horse. Yes, your inside arm will probably want to fall off by the end of it :)
On a side note..
It's so important to reward for lightness. So many riders cling literally on their horse's mouth, trying to "hold" their head down in a fake head set, trying to get them on the bit. That is where heavy horses learn their bad habits, to balance on your hands, and have a weak back end.
So there, my first post on this forum, ande competing to be my longest, is hopefully going to help you.