Lifting the bit into the corners of the mouth (in hand...touching the corners of the lips). As little as it takes to mobilize the jaw and/or change the balance (sometimes that lifting also includes going backward to WAKE UP the mind/reactions). It might be mm or it might be feet. It depends upon the horse. The horse will then REACT (to our actions) and change balance. The handler presumes this, rewards (proactively) the change in the horse. Same thing for mm of lateral flexibility WHILE the horse is upright/high/light. After ANY of these the handler MAY allow the horse to chew fdo....but NOT always, and not IMMEDIATELY after. Again, it is what we allow, not what the horse barges to take.