It is useful to learn how to read horse emotions and not let them get to the point of being worried or fearful all of which can lead to aggression.
Jaak Panksepp is a good source of learning about the 7 basic emotional systems of all mammals.
1. SEEKING - can be a positive or a negative emotion depending on whether the horse is seeking something they want or seeking to avoid something they don't like.
2. FEAR - can be as little as mild anxiety or a full flight response.
3. RAGE - fear can escalate into aggression or frustration if the horse can't escape or get what he wants.
4. GRIEF or panic system may be seen in separation anxiety.
5. CARE - the mutual grooming and nurturing side of horses.
6. LUST - may be seen in the over arousal of clicker trained horses before impulse control is established, or in the normal behaviour of stallions and mares.
7. PLAY - this is self explanatory and something we can tap into when training.
So we can see that chasing horses round a pen (with no means of escape triggers the FEAR system, how much FEAR there is depends on the horse. It may be mild anxiety or full blown flight. Many genres of horsemanship utilise this aspect of a horses natural flight response to train, to gain what they call respect. Yes it may well work to suppress the horses natural reactions but do we really what to use FEAR in training.
Most top level practitioners are experts at reading horses and using flood desensitation - although they rarely admit that is what they are doing. If the horse has no means of escaping the aversive stimulus (other than stopping and trying to appease the trainer) then it is flooding. It may well work if done correctly but the danger is people with less knowledge and expertise copy this and often it fails, failed flooding can make horses more reactive and in some cases dangerous.
We all need to learn what appeasement behaviours look like, I have just bought a book on calming signals. Well worth looking at "Language Signs and Calming Signals of Horses" by Rachael Draaisma.
Until we are good at doing this we often miss some very subtle signs that the horse is uncomfortable with we what we are asking them to do.
This is also useful when training using positive reinforcement and doing liberty work, we need to keep the horse at or under his emotioinal threshold, so not invoking a full flight response nor causing frustration (part of the RAGE system).
PS very few people only use +R but we do aim to use it most of the time. So follow the LIMA principles.