Plankakassan, or the p-kassa, is a fund for fines available to the free riders in the Stockholm public transport system. As a member, the fine you get from the SL controllers for free riding, gets paid by the fund.
It´s all about solidarity
P-kassan is based on solidarity. As a member you help other free riders out by getting their fines paid. When you yourself get caught its other free riders that pay your fine. P-kassan is a way, so that if you cant afford the high ticket prices, you will still be able to use the collective means of transportation through out Stockholm.