I found this online and hope it helps rather then make it more complicated....
Engagement of the Hindquarters
Engagement of the hindquarters is defined as: downward flexion at the lumbo/sacral junction; coiling of the loins.
The following are excerpts quoted from several sources:
US Cavalry Manual -- 1942 (and earlier editions): "The mechanism of impulsion lies in the play of the hip joint (coxo-femoral articulation). The closing of this joint leads to the engagement of the hocks under the mass and allows the horse to cover more or less ground according to the energy of extension of the propellers. Such engagement of the hocks under the mass leads to a lowering of the hindquarters ... the horse must lower his croup and draw his hocks under the mass."
Effective Horsemanship -- Noel Jackson (of the French School of Dressage, learned in Portugal) -- The ramener ("ramener" being on the bit) is the term used to describe the different stages of closing of the angle between the horse's head and neck when he bends longitudinally at the poll. It is complete when the horse's forehead reaches the perpendicular as he momentarily relaxes his jaw. It.... stretches the cervical ligament and the muscles of the dorsal part of the neck and the back, which eventually enables the horse easily to engage his hind quarters and to raise his whole forehand. .. As the horse adopts the attitude of the 'ramener' and tensions his cervical ligament, he gives the rider the distinct impression of raising his whole forehand as he engages his hind quarters, and immediately feels half a hand higher."
Another Horsemanship, Jean-Claude Racinet: "...Engagement of the hindquarters, which the (downward) tipping of the pelvic bone is, and the engagement (or stride under) of the rear feet appear as two distinct phenomena."
Hindquarter engagement starts with coiling of the loins a lowering of the hindquarters, and increased flexion in the joints of the hind leg. In the tolt, the horse contracts his back, therefore cannot flex at the LS joint. There is no sustained lowering of the hindquarters from the lumbo sacral junction, no increased flexion of all the joints of the hind leg, and no bascule of the back -- without that, there is no engagement.
Are the feet (both feet) under the body... or is one behind?
If both feet are working under the body, it helps to discern pelvic tilt.
If one is trailing behind, it's certain that the pelvis is not tilted for engagement.
A quick test to see if a horse is working off his hindquarters in tolt, is to ask for a "stop on a dime" or a roll-back and see what happens.