If I find them starting to get heavy I like to slow it down.
I’ll come back to the walk, maybe a halt. Turn on the forehand, turn on the haunches. Some leg yields. Rein back.
Once they’re soft and relaxed again I’ll start doing some walk-trot transitions. If the horse is running through the transitions I might not move to anything else.
My gelding responds really well to w-t-w transitions. You can really feel him sit on his butt and lift the front end.
At this point I like to add in some lateral movements. If they don’t accept the leg they’ll get strong again.
Spiraling in and out on a circle is a good one, as are serpentines (focus on making sure they’re straight for a couple strides as you cross the center line, the next time ask for a transition every time you cross X).
At the canter do lots of transitions within the gait, lots of circles, counter bend. Counter canter. As you come down the long side use your leg to push them to the quarter line, and then back to the rail again. Canter-trot-canter and canter-walk transitions.
If they stay soft through all of that I’ll add in some pole work. My three favourites are
a) a line of canter poles spaced 9’ apart. They have to stay in a consistent rhythm and pay attention to where their legs are. You can play with this using cavaletti as well, or raise alternating poles, or alternating ends of the poles. It’s hard work for them though, so don’t overdo it. A tired horse will start to lean on you as well.
b) Circle of death. Start with one pole, then two, then four.
c) Three poles 45’ apart. Alternate the strides. Four to a four. Five to a four. Four to a three. A soft, well schooled horse should eventually be able to do a three to a five.