Discretionary Effort; where an animal trained with +R puts in more effort/energy to a learned behaviour than was originally taught with. There is no cap to the amount of effort they are prepared to put in.
This is in comparison with animals taught traditionally. In this situation the animal will put only the amount of effort required to avoid unwanted consequences.
This makes the behaviours (despite being the same behaviour) look different and that difference is what we are aiming for; better cadence, balance, motivation, etc.
The relationship built out of +R training makes the same behaviours look different, better.