p.boxed
  | Captured pieces are stunned. They disappear if you capture them again.

p
  | This variant is inspired by the 
  a(href="https://en.wikipedia.org/wiki/Super_Mario") Super Mario
  |  universe.
  | When a piece captures another, it "bounce" on it until the next square in
  | the movement's direction. If this next square is occupied, then it keeps
  | bouncing and reach the following square, until either a free square or the
  | edge of the board is met. In this last case, the capturer is lost.

p.
  Pieces bounced over are then "stunned" for two moves (four half-moves):
  they cannot move during this period. If they get captured again while being
  stunned, they are "kicked" out of the board, and all pieces standing on their
  way vanish as well.

figure.diagram-container
  .diagram.diag12
    | fen:nrkqnrbb/ppp2ppp/3p4/4p3/1P3P2/8/P1PPP1PP/BNQRNBKR:
  .diagram.diag22
    | fen:nukqnrbb/pps2ppp/3s4/4s3/1P6/8/P1PPP1PP/BNQRNBKR:
  figcaption Before and after 1.fxe5

p.
  After the move 1.fxe5 on the diagram, the red pieces are stunned for two
  moves. So white can then play 2.Bxe5, kicking the pawn and capturing (for
  real) at least the h8 bishop. An option for black may be 1...Qg5, such
  that after 2.Bxe5 Qxe5 black threaten to kick the white stunned bishop out.

figure.diagram-container
  .diagram.diag12
    | fen:nuk1nrb1/pps2p1p/3s4/4B1q1/1P6/8/P1PPP1PP/1NQRNBKR:
  .diagram.diag22
    | fen:nuk1nrb1/pps2p1p/3s4/3qC3/1P6/8/P1PPP1PP/1NQRNBKR:
  figcaption After 1...Qg5 2.Bxe5 Qxe5: white bishop is stunned.

p.
  The goal is to capture the enemy king. Moves which kick your own king
  out are forbidden, but stunning him is allowed.

h3 Source

p
  a(href="https://www.jsbeasley.co.uk/encyc.htm")
    | The Classified Encyclopedia of Chess Variants
  | , section 3.6. This variant is also listed on chessvariants.com: 
  a(href="https://www.chessvariants.com/crossover.dir/koopachess.html")
    | Koopa chess
  | .