Your horse is the ONLY one who can accurately determine what feels stressful, like "too much" pressure, or is perceived traumatic event.
Pressure can come from many sources - environmental (new surroundings, confinement, solitude, dietary, human interaction), direct physical contact, or emotional intimidation or coercion. Pain, physical discomfort, or the strain of being asked to perform beyond conditioning is another form of pressure.
Inconsistent cues, confusion, frustration, and being overloaded are other forms of pressure, or stressors, that many horses experience. When we train with the intention of creating submission we are likely to trigger appeasement behaviors and suppress naturally occurring emotional responses which lead to conflict and further increase stress.
If a horse is behaving in a submissive way they may display hyper-vigilance or withdraw emotionally - going through the motions with a glazed looked, a tense body, or exhibit signs of stress without acting out on them. How we know when our horse is stressed can be determined by their posture and behavior.
Behavior is the window into what is going on emotionally. We need to watch for the most subtle cues. A horse that is "blowing up" has gone WAY over his emotional threshold and is fighting or fleeing for his life. However, it's important to be cautious when assessing behavior and expression. We can only speculate as to what the horse is really thinking. It's best to focus on the behavior itself. You can also make assumptions based generally accepted and recognized stress and calming signals such as: running, bucking, kicking, biting, tension around the mouth, ears to the side, back, moving excessively, or stiffly pricked forward. Eyes at half-mast, wide open, or blinking rapidly are expressions of conflict and tension as is a high head carriage, flared nostrils, and licking and chewing. Any version of flight, fight, or freeze - displacement such as face rubbing, grass diving, hyper-focus, avoidance, non-responsiveness or hypersensitivity are other ways horses express their anxiety.
Calming signals such as looking away are common and often misinterpreted as being "disrespectful." Any interaction that create conflict or set off his survival instincts are stressful to the horse. For example, making a horse run. The discomfort is compounded if he's in a tense unbalanced frame. A horse that is in flight mode is above his emotional threshold. His heart rate is elevated and he's reacting vs thinking. Any escalation of fight/flight/freeze behavior is another sign our horse is highly stressed and at risk of reactive behavior.
If we find our behavior and/or our application of "pressure" is escalating when we are attempting to communicate with our horse, we are falling into the trap of escalating cues, using more force to "get the job done" instead of stepping back and figuring out what is preventing our horse from responding desirably. This scenario is also likely to cause conflict and reactive behavior in your horse and erode his trust. Of course the horse may respond as you like, at which time you remove the pressure. This is a completely valid means of communication, but if the horse is in survival mode when he responds, the horse is having an unpleasant experience, you're getting a limited response, and you're creating an unpleasant connection between you, your horse, and the activity - your relationship and further interactions may suffer.
The horse is the only one who can determine how he feels physically and emotionally. That's why I completely changed to low-stress handling, care, and training techniques including positive reinforcement. It is the least likely to cause distress and has been proven to be the most likely to effectively and efficiently teach, strengthen, and maintain a behavior. I have successfully used positive reinforcement with horses with a wide range of behavior and performance issues as well as deep seated trauma. It has worked to improve the lives of every horse as well as the human who loves them.
As stewards of these magnificent and generous beings, it is vital we learn how to communicate and care for them in a way that preserves their well-being and enriches their lives.