Impulsion is the term used to describe the transmission of an eager and energetic, yet controlled, propulsive energy generated from the hindquarters into the athletic movement of the horse. Its ultimate expression can be shown only through the horse’s soft and swinging back to be guided by a gentle contact with the rider’s hand.
FEI Article 417.3 (2003)

The action of the purely driving leg controls, which are the only influence that can cause engagement of the hindquarters, is manifested by increasing the impulsion of the hindquarters, which begins to carry more of the load and thus relieves the forehand, which now steps out more freely, with longer strides, in harmony with the hindquarters. The neck stretches out forwards and down more and more, corresponding to the extension of the back. This extension is felt in the horse’s reaching out for the bit.
Walter Seunig, Horsemanship

Elastic, energetic steps are an indication of a high degree of impulsion. However there are some horses who produce a trot that resembles a passage but in which the forelegs remain unnaturally straight. It is sometimes termed ‘trot balancé’. This type of trot is not normally a sign of impulsion but is a clear sign of tension in the horse. It can often be an evasion or sign of resistance on the part of the horse to the rider’s forward driving aids. The judge must ask ‘is this really going forward?’ A supple back is a prerequisite for transmission of power from the hindquarters into an athletic stride. There should be no apparent tension in the back. There should be longitudinal suppleness shown by the horse’s willingness to shorten and lengthen his frame according to the rider’s requests and the requirements of the differing paces. Engagement of the hindquarters means that the hind legs are willingly stepping forwards underneath the horse and taking his weight. They are not left out behind the horse’s body.Wolfgang Niggli, Dressage – a guideline for riders and judges