its the clothin line he represents
 Touchdown celebration/endorsement deal
Johnson is known for flashing "The Roc" symbol also known as the "Diamond Cutter" whenever he scores a rushing touchdown. Recently Johnson signed a contract with the Team Roc clothing line founded by Jay-Z. The first appearance of that was after he scored a touchdown in a game on December 4, 2005 (which happens to be Jay-Z's birthday).
“Some people have Nike contracts, Reebok contracts, I don’t have either or,” Johnson said. “I’ve got Team Roc, that’s Jay-Z, Damon Dash... That’s their Nike or Reebok. I’m signed under that.” 
There was speculation that in the near future Johnson may put the symbol "on hold" due to the fact that Jay-Z is currently being sued by professional wrestler Diamond Dallas Page who accuses Jay-Z of copyright infringement, also due to the fact that the NFL has recently strengthened enforcement on endzone celebrations for the 2006 NFL Season. The speculation was put to rest when Johnson resumed using the celebration when he scored his first touchdown in week four of the 2006 season.