BAP is known in the poker world as Buy A Piece. Which means you are buying a certain percentage or share in a poker players action. If that poker player happens to cash in a tournament or tournaments that you bought a percentage in then you receive that percentage back of his/her winnings.
I have bought action on a few players that I know, and it has worked out well, but sometimes it doesn't, and I have had others buy some of my action as well. I usually only sell up to about 25% of my action, but I do know players that will sell up to or even more then 50%.
What I have found most players do is sell their action in a $10,000 Main Event Tournament. They will sell 50% at 1:1.2, meaning for 1% of their action will cost you $120. So if they sold 50% that would be $6,000, then they would take that $6k and and buy into the smaller side events, where they get all their own action.
Not everyone does this but it is common place for this to happen, and is a pretty good way to be able to play a lot more events then just The Main Event.
Some players also swap percentages when they are playing in the same events. This is done evenly on a one to one basis, and is also a good way to increase your bankroll.
Every player is different with what they will sell and what they will swap, and sometime things can get confusing. If you decide to sell or swap your action, my advice is do it with someone that is trusted or you know personally, and keep it simple so their is no confusing when the time comes for you to receive or payout what you owe.