It's nice when your horse understands English. Harley was being lazy, and I couldn't ride long as my back and foot hurt...plus it was humid. I stopped by the door and looked to see if I could spot anyone in the barn, I called out, "Hey Kim, if you get a minute, could you bring me a crop". She said, "Sure". I then asked Harley to trot. Harley is a smart boy. He understands many English words. Crop is one of them. He took off at a very good pace. I still took the crop when Kim, brought it to me, but I never had to use it. Just knowing I had it (even just knowing I could get it, apparently) was all the motivation Harley needed. He kept a great trot and cantered well-even doing a beautiful simple lead change and cantering circles without breaking. After our workout, I gave him his first bath of the year, then let him graze a bit. He also got a treat. He is a good boy, even if I do have to threaten him at times.
For any non horse people-using a crop properly does NOT hurt the horse. As long as it is used properly, it is just an extension of your leg, giving a cue to go faster, or a tap to get his attention.