I am utterly obsessed with it Like in a weirdo OCD way! I vigorously brush main muscle groups everynight (if worked in the morning or vice versa)!! In my head it prepares the fibres for more body building the next day! Seems to work though. I have strict Pessoa regimes and I try to push for overtrack all the time. The Mare in the bottom picture took slightly longer as she had a lot of muscle on the underside of her neck. Freckles (top pic) needs more cover over his rump so hills at every opportunity I can get. Polework and grids help too
The other thing I found that helps is lunging with loads of transitions (and I mean LOADS - physio told me to do trot to canter, 3 or 4 strides of canter, back down, straight back up etc) and asking for the horse to really "sit" as he pushes into the next pace. Makes a big difference and IMHO is much more useful than just trotting and cantering around endlessly (which you see many people doing).