Whoop! I'm back with a little question myself, too.
I noticed today while doing bending and marching exercises at walk that my mare stumbles and trips after doing a large circle to extend her stiff side. Wondering if that's something normal when working like this... Whether it means I've done something right or incorrect. She likes to overbend her neck, so I started using my outside leg and hand to limit her neck bend but still keep the overall bend and step-under. Felt great, until after the circle is done... Not falling down, but catching herself and wanting to trot.
Ahem. For me, I practice the stride lengthening when my horse feels comfortable stretching out, and stride shortening when we do our bending. It's easier to bend frame-by-frame slowly as molasses and add some more impulsion later. Lengthening, would be opening fingers a bit and pushing the horse on to reach, reach, reach, not just with the feet, imo, but with the whole body swinging. Shortening is the half-halts, one after the other, til you get it, or even downward shifts.
I think it's easier to shorten during bending because you should already be riding the horse into the outside rein, so capturing energy going to it is straightforward.
And... when in doubt, trotting poles out ;) When I feel particularly evil, I work my horse in a sandier area with a few trotting poles on one side. She gets the consequence of hitting the pole when she goes at the wrong stride or wrong speed, so there is more a chance of the horse learning by its own movement than by you directing it. Something you could do is put a short-strided trotting pole set on one side of the arena, and a longer-strided trotting pole set on the other, or setting up a circle to be lengthened in one half and shortened in the other. That will definitely get her booty moving x)