It's like being tossed into the fire.
There's no gradual climb in intensity, no time to let the rookies adjust after the all the craziness of the predraft testing and interviews, the draft itself and the signing process. Pro coaches bring it from Day 1.
The transition happens so fast that the overall tempo can sneak up on rookies. The sessions are high-speed and designed to test conditioning levels. Players are asked to change direction, drive on the ball, break down in the open field, etc. I'm talking about functional football movements that these guys haven't done for months while they've been training for 40 times and vertical jumps.