I can reinforce him. What I am doing is drawing attention to that subtle change so we can both become more aware of it. This is a wonderful process and one that can become quite addicting. Think about a child scratching out his first few notes on the violin versus an accomplished musician. That's the difference in the level of response we can get from our horses.
Many riders never go beyond asking for gross responses.
One reason for this is they simply don't know how much more their horses can give them. They become satisfied too soon, and the thought of all that repetition puts them off. Their idea of repetition is an assembly line stamping out exact replicas of a behavior until the esponse becomes mindless, automatic, and yes, very boring. With that kind if expectation these riders often miss all the wonderful, but often subtle changes that are occurring in their horses. They have not been trained to observe them.