p.boxed.
  White has only four pawns and the king, but move twice at each turn.

figure.diagram-container
  .diagram
    | fen:rnbqkbnr/pppppppp/8/8/8/8/2PPPP2/4K3:
  figcaption Standard deterministic position

p.
  The white army can appear much too small, but the power to move twice in a
  row shouldn't be underestimated. At each turn white plays two moves with
  only one constraint: do not be under check in the end.
  So if the white king attacks a defended piece, he can take it anyway by
  coming back on its initial square on (sub)move 2.

figure.diagram-container
  .diagram.diag12
    | fen:rbbknn1r/1p2pp1p/2p3qK/p2p2p1/2PPP3/8/5P2/8:
  .diagram.diag22
    | fen:3rq3/1p4p1/1k1pKp2/3P1P1n/p7/5n2/8/8:
  figcaption Left: not a checkmate! Right: a "Monster-checkmate".

p
  | The diagram position on the left looks pretty much like a checkmate,
  | but white can take the queen and come back to the h6 square. Finally,
  | white can mate in an unusual way, like the following diagram found 
  a(href="https://en.wikipedia.org/wiki/Monster_chess") on Wikipedia
  | . There is no way for the black king to avoid being captured since white
  | plays twice (the threat is 2.d7,dxe8).

figure.diagram-container
  .diagram.diag12
    | fen:4k3/8/4P3/8/3P4/8/2q5/7K:
  .diagram.diag22
    | fen:4k3/8/3PP3/8/8/8/2q5/7K:
  figcaption Left: before 1.d5,d6. Right: after this move:, it's checkmate.

h3 More information

p
  | Ralph Betza analyses this variant and the double move on 
  a(href="https://www.chessvariants.com/d.betza/chessvar/muenster.html")
    | this page
  | . There seems to be a common belief that black should win with accurate
  | play, but it's clearly hard to demonstrate. And if someone can show a
  | winning strategy, we'll add some white material to balance this game.
  | Meanwhile, the variant is also playable 
  a(href="https://greenchess.net/rules.php?v=monster") on greenchess.net
  | .