Calhoun nailed it.
Reddings Guide has a good explanation, which is why buying one is a good investment:
PSK Fouls: "Post scrimmage kick enforcement allows the receiving team to retain possession of the ball after they have forced the opponents to punt, even though (they) commit certain fouls before possession actually changes....."
.....Under (PSK) enforcement.....the all but one principal is still used but it is applied as if ...R were the team in possession."
4 requirements: 1) Foul is by R. 2) Foul, (other than Ill. Sub. or Ill. Part. fouls occurring at the snap), occurs beyond the ENZ. 3) Foul occurs from the snap until the end of the kick ( but not kick tries or a successful FG) and the kick crosses the ENZ. 4) R will be next to put the ball in play.