Teach Yourself Bagpipes by Lindsay Davidson

Crossing sounds... 

A 'crossing sound' is the additional, unintended sound that happens when changing from one note to another and the fingers make an unplanned position. This is one of the worst mistakes in 'piping and is to be avoided at all costs.

 We avoid crossing sounds by making sure that we know which finger causes the change in sound to the note which we are aiming to play (i.e the thumb makes the high sound or not sound) and when going to this note, move this finger first (when going upwards to the note from another) or last (when going downwards from another note).

slowly with a crossing sound (this is how NOT to play!!!!)

slowly and correctly, with no crossing sound

quickly (normally) with the same crossing sound (this is how NOT to play!!!!)

quickly (normally) and correctly without the crossing sound