Clarification of NFL's PUP Rules

Now that NFL rosters have been cut down to 75, many of the most recent moves involve placing players on the Physically Unable to Perform (PUP) list. Here's an explanation of that designation:

Players must miss the first six weeks of the regular season. After Week 6, whether a team has had a bye or not, a 21-day period begins where a player can begin practicing. He can start practicing at any point during those 21 days.

The player could be activated at any time within those 21 days. At the end of the 21 days, the team must either activate the player, release the player, or keep him on reserve/PUP for the remainder of the season. Note: The player would remain on reserve/PUP, not go on injured reserve.