He stood on his combine numbers. Here is a blurb on him from NFL.com:
"One of the best players was safety Rashad Johnson (5-foot-11, 200 pounds). He was not always considered to be among the best, however. He had to walk on at Alabama just to make the team. Johnson has now turned himself into a fairly high draft choice through hard work. This just goes to show there’s room for all walk-ons to become stars.
He did not do the workout drills Wednesday, standing on his numbers from the combine. He did perform all the positional workouts."