Taken from the wiki

In Brawl's co-op Home-Run Contest, it is possible to view the area to the left of the main platform. This can be done by having one player hit Sandbag off the platform, then having the other player send Sandbag flying to the left of the stage with the Home-Run Bat; the camera will continue to follow Sandbag, showing the backside of the stage, as well as its "end" if Sandbag is hit far enough.