I have a horse that is very weak behind and I've been working on multiple things with him over the past few weeks. When I'm flatting him I always try to keep his head higher because this engages his hind end. It's not always necessary to keep their head in a frame tucked into their chest. They actually use their hind end more when their head is raised a bit. I work on a lot of lengthening and shortening in the canter as well as downward transitions from canter to trot. A few days a week I will set canter and trot poles out and work with him on those as well. It also does help to do uphill work on hacks!