He actually was working on his hands.
While he was on the ground, Cobb caught pass after pass from the JUGS machine on the sideline.
Although the machine isn't visible in that picture, it was positioned to Cobb's left, perpendicular to his body.
He said it was something he started doing this offseason to help with over-the-shoulder catches.
"I'm not always going to have the ball right directly here [in front] like the JUGS machine shoots," Cobb said after practice. "So I have to work different angles. It's whenever I'm looking back for a ball, coming over my shoulder, you have to work different angles when the ball comes in."