Your supposed to ride with your thumbs up. (its not a stupid question, a lot of people don't know it.. and a lot of people ride with their thumbs up without knowing why) Just like you hold western (rommel reins) with your thumbs up.
It makes a clean, straight line from elbow to bit. The uninterupted connection from bit to elbow provides smoother communication and the horse readily understands what you are asking him. If you ride with your thumbs in the line from elbow to bit is most likely broken making the communication less clear and choppy.
You should keep your elbow in and at your sides (if it goes behind your back, your reins are too long) for the optimal elbow-bit line.