Classical conditioning means associating one stimulus with another one. It dates all the way back to Pavlov’s experiment with dogs, where ringing a bell caused the dogs to salivate because it predicted food. 

Horses are very good at this type of association, as long as it happens within a short time frame - less than 3 seconds. This makes timing very important. 

An example. You have taught the horse to step back when you tap it with a whip on his chest. Once the horse knows this well, you can teach the same behavior from a voice cue. When training it, the voice cue needs to happen immediately before the tap with the whip. 

Similarly, after we have taught the horse to stop from a rein aid, we can train it to respond to a seat aid. We must use the seat aid just before the rein pressure. It’s important to make sure stopping from the rein aid is very well trained before you add the classical conditioning.