underflip and rodeo are two completely different tricks. Rodeo you just lay on your back and set as if you're setting a lincoln. Underflip is set a lot like a cork 5 but inverted-- not a misty though, more like a d-spin 5 if that makes sense. Another way to think about it (my way of thinking) is to think of it as a quick 180 into a frontflip, i know this might sound crazy but it helps me. Also Rodeo is a much easier set
for underflips my trick is set a backflip and once you're upside down just look over your shoulder. if you set your flip knowing you're going to look over tht shoulder, you should start the flip off axis a bit. basically, your head should be doing an upside down J to the left motion.