Complex sentence: In the Swaps, Seattle Slew, who normally broke on the lead, was unable to get to the front, as Bill Shoemaker sent J.O. Tobin, whom Seattle Slew had easily beaten in the Preakness, to the lead.
Split into simpler sentences: In the Swaps Seattle Slew (who normally broke on the lead, as noted above) was unable to get to the front. Jockey Bill Shoemaker sent J.O. Tobin (whom Seattle Slew had defeated in the Preakness) to the lead.