In a Progressive Knockout Tournament, each player has a bounty on their head which increases with each elimination. As a result, as the tournament progresses, the bounty on your head increases, making it more and more lucrative for other players to try to knock you out.
If you knock someone out, you get half of your opponent's bounty as a cash prize immediately. The other half goes into your bounty, much like with a regular PKO tournament. The obvious major difference is that there's no such thing as a bubble, or a regular prize pool. All the money awarded goes in bounties.