It's when you're fucking a girl doggy-style and then you whisper in her ear and say something like "I have herpes!" and she'll try to get away. You will try to hold on to her waist as long as possible making it seem like you're bull-riding!
Guy: "I have herpes"
Guy: "Yeeeehhaawwww!!" (I'm so gonna tell my friends how awesome it is to be a Texas Bullrider!!)