In a PKO (Progressive knock-out) tournament, a bounty is placed on each and every entrant in the tournament. Each time you eliminate somebody, you win an instant cash prize! Every player will start out with a bounty (half of the buy-in), and eliminating a player will give you half of their bounty as an instant prize, while the other half is added to your own bounty.
For example, in a PKO event with a $20 buy-in, everyone will start with a $10 bounty, and the remaining $10 goes into the regular prize pool. When you knock someone out with a $10 bounty, you will take $5 into your account, and $5 will be added to your bounty.