The connection between yielding to pressure and collection is direct and foundational — collection is, at its most fundamental level, a horse that yields its hindquarters under its body in response to rein and leg pressure and carries itself in a more uphill, balanced posture as a result. Every degree of collection achieved in a trained horse represents a refinement of the yielding response: the horse has learned to yield its hindquarters more promptly, more deeply, and from lighter and lighter cues, until the degree of engagement required for collected work happens from an almost invisible request.
Lightness — the state in which a highly trained horse responds to barely perceptible cues with maximum expression — is the endpoint of progressive refinement of the yielding response. A horse becomes light not because it was born with a light mouth or an unusually sensitive body, but because years of careful pressure-and-release training have taught it to search for and respond to the lightest possible version of each cue. The trainer who always escalates to strong pressure before the horse has had a chance to respond to light pressure systematically teaches the horse to wait for the stronger cue — which is the opposite of lightness.
Al Dunning's work with Western performance horses, and the classical dressage tradition more broadly, are both built on this understanding. The collected reining horse that can slide-stop from a barely visible change in the rider's seat, or the dressage horse that moves into piaffe from a whisper of leg, achieved that responsiveness through the same principle: light pressure, immediate release on the smallest try, progressive refinement over many sessions until the try became the full response and the full response became available from a lighter and lighter cue.
The practical implication is that trainers who want light, collected horses must train for lightness from the beginning — rewarding the lightest responses to the lightest cues, never waiting for the horse to push through a half-hearted aid before responding, and releasing completely when the horse gives rather than maintaining contact to demand more. Every session is either building toward lightness or teaching the horse that heavier pressure is normal.