GarageBand '11: cycle region
Using the cycle region, you can record over a specific part of a project and record multiple takes. To define the part of the project you want to record over, you turn on the cycle region and then drag the ends of the cycle region in the area below the beat ruler. As you record and play the project, the playhead returns to the start point of the cycle region each time it reaches the end of the region.
To turn the cycle region on or off, click the Cycle button in the control bar.