What are the most important things to watch for in a horse's body language during groundwork?

Reading a horse's body language during groundwork is a skill that Clinton Anderson, Pat Parelli, and Warwick Schiller all teach as foundational to effective training, because the horse is communicating constantly about its emotional state, its level of understanding, and its willingness — and handlers who miss these signals either push too hard or release too early. The most important positive signals — the ones that indicate the horse is processing, learning, and beginning to relax — are licking and chewing, blinking, lowering the head, a slow exhalation, and the inside ear turning toward the handler. Schiller is particularly specific about these processing signals, which he teaches are signs that the horse's nervous system is regulating downward from activation toward calm. These signals indicate the horse is in a state where learning is happening. The most important caution signals — the ones that indicate the horse is moving toward overwhelm rather than toward learning — are a fixed eye that stops blinking, a high, tight head, flared nostrils, held breath, sweat appearing rapidly, a tail clamped against the body, and rapid switching of the tail. These indicate the horse is in a reactive state rather than a thinking state. Parelli teaches recognizing which Horsenality type is showing these signals, because the intervention differs. Anderson emphasizes watching the horse's hip and hindquarters as the most reliable indicator of where the horse's attention is. A hip cocked toward the handler is a submissive, relaxed posture. A hip angled away with weight shifted toward a hindquarter indicates potential kicking. Hindquarters that are constantly moving suggest an unsettled, forward-leaning attention. All three trainers teach watching the horse's relationship with the inside of the circle during longing. A horse that consistently drifts to the outside is looking for an exit and is not mentally engaged with the handler. A horse that consistently cuts in is looking for closeness and has a good relationship dynamic. The quality of the horse's circle geometry tells the handler a great deal about the quality of the connection.