You guys know I'm all about the natural horsemanship thing which everybody thinks is lovey dovey please be a nice horsey. No. I always have a whip handy. At first it's for my own protection. I've been charged at with bared teeth and striking hooves while doing groundwork. They get a good snap on the nose if they try that. If I don't need it, great, rather have and not need than need and not have.
While they are learning to drive, the chirp is the first cue, then the whip is first tapped on the ground getting closer and more forceful. I usually don't have to actually hit the horse, but will if necessary. Once I get the desired response (in this case forward movement) the whip is immediately put back on my shoulder. When I'm sure they know what I want and maybe now they're testing me*, it's please, pretty please, (whack!) pretty please with "sugar". No playing around, I said "move" I mean move NOW! Doesn't take them long to learn it's in their best interest to respond at the first ask.
So, carrying that over to the race.... if a horse has been properly trained to respond to the chirp, the whip shouldn't really be needed. Certainly not used to slash unmercifully when the horse is giving you all he has.
*I've never had a horse NOT try to test me at some point. Almost ALWAYS on the fourth day of training. I look forward to those days....