p.boxed.
  At each turn play first a normal move, and then exchange two pieces'
  positions by making a capture.

ol
  li.
    Each turn is made of a standard move and a swap move (see 2).
    At the first white move, only the regular move is done.
  li.
    A swap move is a two piece swap of positions. One piece is a player's
    piece, and the other is on a square where the first piece could move
    (if the square was empty) or take (if the square had an opponent piece).
  li If a player cannot make the regular move or the swap, he loses.

p.
  On the diagram the queen first takes the g7 pawn, and then swap its
  position with the g8 knight: checkmate.

figure.diagram-container
  .diagram.diag12
    | fen:r1pqk1n1/pn1pppp1/2p5/8/8/2PPPK2/BP4Q1/N1R2P2:
  .diagram.diag22
    | fen:r1pqk1Q1/pn1pppn1/2p5/8/8/2PPPK2/BP6/N1R2P2:
  figcaption Before and after 1.Qxg7,Sg7g8#

h3 Source

p
  a(href="https://www.chessvariants.com/diffmove.dir/balancedswap.html")
    | Balanced Swap Chess
  |  on chessvariants.com. Here is an 
  a(href="/variants/Swap/game.pgn") example game
  |  played by the variant's creator.

p Inventor: Joao P. Neto (1998)