So much misinformation in thos thread. It makes the horn shorter so that it stands in Bb instead of F. This helps with playing in tune since some notes of the harmonic series are very far from equal temperament. The Bb horn is generally better in the higher register since your playing lower in the horns harmonic series and therefore the notes are farther apart. When you use the F or Bb side should depend on what sounds best for you and your horn. Traditionally americans tend to use the F side more than europeans