Complex sentence: Turcotte later said that he was proudest of this win because of the split-second decision he made going into the turn: I let my horse drop back, when I went to drop in, they started backing up into me.
Split into simpler sentences: Turcotte later said that he was proudest of this win because he had to make a split-second decision after the field started slowing down going into the turn. When I went to drop in, they started backing up into me.