Great shot Tim. I've been trying a few of these myself lately. Haven't been able to get great light on my subject like this though. Clarity on the horse and the jockey's face is great. Also you've managed to get a nice plain background. Well done. Would have been perfect without the 2nd horse there competing for attention. I would be interested in your knowing your shutter, aperture and ISO used.
Sheesh, a 7-legged horse! It's bizarre to look at... In other words, the near-perfect overlap of the two horses is producing some disconcerting effects.
Fine image of the power of the animal - and perhaps the best mid-stride pose one can get, showing the full extension. I'd perhaps have preferred the exposure a little under this point - the whites are just blown, and and easing of the overall sense of brightness might give more tonal balance to the whole image.