Bridging

From David Lichman:

A secondary reinforcer is a sound, touch or action that signals to the horse the exact moment he did something correctly. It's called a "bridge" because it bridges the gap in time between when the animal does the behavior correctly, and when he gets his primary reinforcer (reward). 

The bridge, over time, becomes reinforcing in and of itself, and you don't need to use the primary reinforcer every time. In fact variable reinforcement schedules and amounts are more powerful. 

You can also have a version of the bridge that means "keep going" or "you are getting close." The bridge itself would be sharp or crisp - the keep going signal, or "intermediate bridge" would be more drawn out and said with an encouraging tone. If Jennifer Maxwell's horse knew this, she could encourage him as he's coming towards her - "good, good, good... you're getting it!" And pique his interest.