I don't have Garageband but I have Logic Pro 9, and they are very similar. I don't know if it has the same function, but in Logic it's called Flex. It's setting for Tempophone will stretch or shrink audio files. It's located next to Automation on the top of the interface. You click Flex and where Volume and Pan icons normally show below the track name it will say off. Click it and it will show the functions, Tempophone is next to the bottom. You click the track at the beginning and then the end, then drag the end. You can't go too far before it gets artifacts. You can also use it to adjust notes that are off beat in a riff(my fav.). There are lots of youtube videos on GB and Logic.