The hand in the bell is used to keep the horn from going sharp, which without said hand it would tend to do. There are also other lengths of tubing you could add to make certain registers easier to play, ie, a Bb side is standard on most horns to faciliatate notes above the staff. The horn is probably one of the most difficult instruments to play, owing to its tiny initial bore size and, as a function, closeness of notes on the harmonic series.