Center of Costume is a video in a series of video tutorials under the Help tab. It describes how to make smooth lines in the Scratch Editor. The video is 32 seconds long.
[0:00] Each costume has a center that it rotates around
[0:04] You can see and change the center by clicking on the "costume center" tool
[0:11] To change the center, click where you want the center of your costume
[0:18] The costume center will now appear in the middle of the paint editor window
[0:22] This can be used to align costumes for an animation or change the center for rotating (demonstrate)