If you were talking to a trainer and he mentioned disengaging the hind end in respect to riding, run as fast as you can. No WP trainer will tell you to do that--not even the bad ones.
However, there is a lot of talk about it in western in respect to lunging and leading. This has nothing to do with collection, and everything to do with control over the horse. The horse's power comes from his hind end--if he needs to run away from something, kick something, that comes from the engine. If you tell him where to put in, and swing it off balance, the horse finds he has to listen to you. Getting control of this will make a lot of 'unruly' horses better in just a few moments.
The problem with getting the true lope and true jog is that WP trainers tell you to slow down to collect. In reality, you collect to slow down. This changes the fundamentals of riding the WP horse--you have to work him FORWARD FORWARD FORWARD, because you CAN. NOT. Have collection without impulsion (forward relaxed, controlled movement). That is the push. Then, and only then, can you work on the 'carry', which also will slow them down.
Slowing them down (pulling back on the reins, 'pushing them into your hand' by squeezing with your legs and holding with your hands, slowing your body, checking), before the horse has the muscles to go forward, will result in a horse on the forehand 100% of the time.