After a very tall man has sex with a male or female horse (the man taking part in the role of the "giver"), he takes his dick out of the horse's asshole/vagina and sprays his cum on top of the horse's back. Before a horse race will begin, the lucky horse jockeys will notice said stain, and hence refer to the stain as a "Cum Jockey". The unlucky ones will place their saddle on top of the Cum Jockey, thereby causing the Jockey to slide around and be imbalanced during the horse race due to the lubrication effect of the sperm.
Jockey 1: Hey Fred, don't sit on Seabiscuit just yet! It may have a Cum Jockey on it.
Jockey 2: This must be the work of... Tall People! >:O