“If the carrying capacity of the quarters is developed sufficiently, the horse is able to carry his own and his rider's weight in perfect balance.”
“To examine whether a horse is in balance or not, the rider can be asked to 'give and retake' the reins, as is done in some dressage tests. Then, for a few strides, the rider has to give up the contact, which anyway at this stage of training is only light. Momentarily without any contact, a balanced horse will not change tempo nor change his head and neck carriage.” The Principles of Riding

Collection cannot be obtained without impulsion, and this may require some further explanation. The rider already working at this advanced level will not need to be reminded that useful collection, which is collection that retains its element of impulsion, must be obtained from the back to the front. The bringing of the hindlegs a little further under the body mass, a little closer to the front legs, will never occur if the horse is at rest or idling. In the horse’s natural state, without a rider, collection will occur only if he is subject to some external stimulation such as fear or excitement. It requires a degree of energy and activity quite beyond normal. In ridden work, when fear and excitement are absent, the necessary stimulation must emanate from the rider.
Anthony Crossley, Advanced Dressage

A horse that does not go well upon his haunches, can never do well in the Manege so that our whole study is to put him upon them.
William Cavendish, Duke of Newcastle,
A General System of Horsemanship

You increase the activity of the trot and keep the cadence. You then start to shorten it as if you wanted to trot on the spot. Use your torso and waist, not the reins. In this way the piaffer is a balancing feat and not a forced restraint from which the horse is trying to escape.
Nuno Oliveira, Horse & Rider: Annotated Sketches (with Jean-Louis Sauvat)

The difference between the various tempos – the ordinary trot, the collected trot, the extended trot – does not lie in the acceleration or the reduction of the pace, but exclusively in the lengthening of the stride or the elevation of the steps while maintaining the rhythm.
Alois Podhajsky, The Complete Training of Horse and Rider