Response versus Reaction

The farrier arrives, and I bring Gally in for shoeing.

"Clancy, I need your help", I announce.

We talk about my problems with Gally, and Clancy (who is well versed in Monty Roberts and Pat Parelli and other horsemen) has a bit of a play with Gally. He holds the head stall with one hand, and tries to get the horse to step away from him by poking him in the ribs with the other hand (the pressure increases as the horse doesn't respond). In the end Gally cow kicks, and then moves away from the pressure.

"Did you see that?", says Clancy, seeming very pleased. I gaze somewhat bored. Of course I saw it.
"Yes", I respond politely.
"The cow kick, was a reaction", says Clancy. "The moving away was response." Pause. "I wouldn't worry about the kick. As he learns to respond, that cow kick will just go away", he concludes.

I wake up from my bored gaze. Eureka! This is it! This is that bit of information I was missing, and I didn't know how to ask about it, or what it was that I was asking about. Reaction versus response.

Most horses will react in some way. Most horses react by running away. And this makes them look like they are easy to train. They are never perfect, but kind of close enough to good enough.

A response is when a horse actually understands the command. This is what most people are searching for.