I'd be interested to here some opinions of some other WP and reining riders.
I have certainly seen lots of WP horses moving with their hind ends behind them both at the jog and the lope, though it is entirely possible I haven't seen a good or correctly trained one.
Oh, and let's define our terms, as engagement has different meanings in different disciplines. In this context, I am refering to engagement as how far under the body the hind leg travels. Dressage horses and hunters must at least track up or having signifigant overtrack. So when I say WP horses disengage the hind end, I mean the hind leg doesn't come up under the body or come close to tracking up. I would be very hard to produce the WP jog or lope if they were.
I am the first to always say there are poor examples of WP horses. So I do absolutely understand where a lot of the misconceptions have come out from.
However, to get a true lope, and true jog...the horse DOES have to be using itself properly from behind. Now, you may not see them overtracking exactly (Charlie our DWB/TB does this just at a walk...hence his ability to pull shoes in pasture!). However, they doesn't mean they aren't truly engaged from behind.
We don't want slow and short. We want long, but slow strides. As my trainer says, she wants to feel the hocks slap her as she lopes them. When I ask for collection in my horse, I am saying....Hey, lengthen your strides, but don't speed
up. Similar to a half-halt.
This was a video taken of my 8 year old WP gelding on his second day of learning lead changes (don't mind his first two attempts, he was fresh into his lesson). He wouldn't be able to switch so smoothly if he was not using his hocks properly.
AQHA...In terms of headset, don't worry about it. Too many people worry about where the horses head is, and they cause a 'false frame'. The head is down and 'rounded', but the back is down, and they aren't engaged from behind.
If you are pushing your horse properly from the hind, they will naturally relax into the bridle. Now where they go depends on the horse.