You should give impulsion to your horse when you feel he's slowing down, before a jump and between jumps if he needs it. My trainer told me to rub my heels up and down on the sides of my horse and roll my upper body in the saddle to urge him foward. :)
A trainer told me that when I was approaching the jump, and then I don't charge at the obstacle!
Yes, I do it in a canter, as it my horse stills sensitive with my legs, and it helps me to feel more my horse :)