Simply put it is pushing the horse into the hand with a momentary closing of the hand, this increases the collection and attention of the horse. A hollow back is a sagging or depressed back usually caused by slackness or constrained contraction of the back muscles. A horse with a hollow back lacks springyness, swing and elasticity. All of these terms refer to the quickness of the tempo at which the horse is moving. Tempo needs not only to be rhythmic but must also not be to fast or to slow, quick, hurried or rushed.
Impulsion is thrust, the release of stored energy into engagement.
Impulsion really does not apply to the walk since suspension must be present to achieve impulsion. There is movement forward in the trot and canter, where in both gaits there is a moment of suspension which is accomplished when impulsion is present. The side of the horse that is toward the center of the ring. Impure, unlevel or un even. This can be momentary or pervasive, and and may or may not be due to unsoundness.