Movable gobos (a type of baffle).
This is a year-old article that has some really nifty inside bits about how the BBoys created "Ch-Check It Out." The rest of the piece is a bit annoyingly celebratory for an album that nobody has listened to since it came out, but the anatomy of the song part is worth a read.
Why am I just now finding and posting this? Because Mike D rocks a black stroller.