p.boxed
  | You have two new pieces in a pocket, which can appear after any
  | first piece movement.

p.
  When a piece is moved for the first time, it is possible to place a new
  piece at its initial square.
  You have two pieces that can enter the game like this:
ul
  li The Hawk (H): moving like a bishop + knight.
  li The Elephant (E): moving like a rook + knight.

figure.diagram-container
  .diagram
    | fen:rnbqkbnr/pppppppp/8/8/8/2N5/PPPPPPPP/REBQKBNR:
  figcaption.
    Position after 1.Nc3/E. White moved his knight from b1 to c3 and placed
    the elephant on b1.

p.
  After castling, a new piece can be put at the initial king square
  (if not occupied), or at the initial rook square - but not both.

p.
  If a move let the king under check, then it is forbidden,
  even if placing a pocket piece would have covered the check.

h3 Source

p
  | Some explanations about the rules and a link to buy a physical S-Chess set
  | on the 
  a(href="http://www.seirawanchess.com/") Seirawan Chess page
  | . This variant is playable on 
  a(href="https://www.pychess.org/variant/seirawan") pychess-variants
  |  with an engine available for analysis. See also the 
  a(href="https://en.wikipedia.org/wiki/Seirawan_chess") Wikipedia page
  | .

p Inventors: Yasser Seirawan and Bruce Harper (2008)