Re-tying the String

If you have a rotary valve on your trombone, according to my in-house player, it allows you to go from a B-flat to an F just by flicking a lever with your thumb, otherwise you have to extend the slide all the way out to reach that F. This is probably a gross over-simplification, but there was nothing overly simple about re-tying the yellow string that makes it all work. 
The string had come undone, and we both worked on it today - the main accomplishment of the day.