First I want to describe impulsion as some people think it means running the horse off it's feet. A horse that shows impulsion is one where you can ask for and IMMEDIATELY get, and gait at any time. If you're cantering and you ask for a walk or halt the horse with impulsion steps underneath itself with it's hind legs and walks/halts as smooth as butter. Why? Because the hind end (butt) is activated and stepping underneth it's body so that it carrys weight.
The higher up the dressage level you go the more it becomes carry, at the lowest levels it's more like push.
So if I'm walking and ask for canter (correctly) and the horse trots? I did NOT have enough impulsion, so I have to use my legs and seat to drive the horse forward WITHOUT ruining the rhythm of the walk (i.e. the horse's walk doesn't speed up - unless it going VERY slow).
You get impulsion from activating the hind end. Once you do that the horse will try to send the energy forward and may trot (instead of walk in the walking example). So then you must channel that energy OVER THE BACK of the horse into the reins so that the horse is marching in a steady forward rhythm - no running, no slowing down - just a steady beat.
Impulsion is at W/T/C and rein back. Without it horse is strung out, too slow, and/or too fast.
best way is watch videos of less experienced riders versus top dressage riders (sometimes at the same show in the same class), an experience eye can show you the difference between them. After you see the difference then experiment on the horse and see of you can feel the difference.
<-- See the horse's picture (in my avatar?) That horse is coming nicely forward from behind over his back (see neck?) and trotting up hill - THAT shows impulsion!