I'm terrible at explaining this stuff. You do it for a few years and it becomes second nature haha. Theoretically, you follow the training scale. Rhythm, relaxation/suppleness and finally contact. The rhythm is not chasing the horse around the arena, that is very "Old German", but it is not plodding along at the pace which the horse chooses. The rhythm is a clear, 4 2 or 3 beat gait that shows natural desire to move forward and has room for a little slowing down or speeding up for a few strides. The horse is balanced. To create suppleness/relaxation, we ask the horse to respond to the leg and move a little more forward, respond to the balancing and slowing aids and come a little slower. This begins to expand and contract the longitudinal frame. We start to take flexions inside and counterflexed, giving in between (Ie, flex left, give BOTH hands, flex right, give both hands) and keeping with the slight forward and back. As the horse's rhythm is settled into and the slight suppling both longitudinally and laterally start to flow, the horse begins to drive from behind, through a soft, relaxed body and into the contact, which is regularly giving to encourage the seeking of the bit. From here as the bridge of the back is built and becomes stronger the horse starts to develop the next step on the training scale, impulsion. Now this ends the warm up phase and we work on keeping this in a shorter frame, and work on straightness and finally collection. All with the first three steps still in place. The contact is the same feeling whether the horse is piaffing or stretching long and low.