p.boxed
  | In addition to standard moves, a piece can be exchanged
  | with an adjacent friendly unit.

p.
  Instead of a normal move, a piece may be exchanged with an adjacent
  friendly one. Switching can move pawns until rank 1 or final rank.
  In the first case a 2-squares jump is possible, and in the second a
  promotion occurs.
  Switching must involves two different units.
  Switching while the king is under check is not allowed.

p.
  Notation: a switch move is marked by the capital letter 'S' followed by
  the exchanged squares. Example in diagram: Sb8c8.

figure.diagram-container
  .diagram
    | fen:1RB3k1/5ppp/8/8/8/8/8/K7:
  figcaption White can mate in 1 by switching b8-c8

p.
  Note: to trigger a switch move involving the king, select the non-royal
  piece first (to not interfere with castling).

h3 Source

p
  a(href="https://www.chessvariants.com/diffmove.dir/switching.html") Switching chess 
    | on chessvariants.com.

p Inventor: Tony Quintanilla (2004)