A multiple displacer (MD) is a version of a piece that can capture enemy pieces partway along a longer move.
The easiest example to illustrate is the MD Rook. Assume a FIDE array but with MD pieces. This has the alternative fools mate of b2-b4 a7-a5 b4xa5 a8xxa1#. The power of the MD Bishop is demonstrated if White starts e2-e4, as Black MUST then move the e7 Pawn to prevent f1-b5#. The MD Queen combines the two. this is what stops c7-c6 being a defense by threatening c6xb5, as White has the alternative of d1-h5. Indeed this works well even if Black has played e7-e5 as it leads to h5xxh8 and, whatever Black plays, h8xxh1. As this gives White an easy win, a game in which multiple displacement is general should have either no modern Queen or some extra restriction such as a River that Queens cannot cross. On a 3d boards MD Unicorns and compounds with them are possible.
There is no MD Knight as the Knight does not pass through the centre of an intermediate square. There are however MD versions of the Nightrider and Rose, which make successive Knight moves capturing on their landing points if required. The MD Rose could start and end a move on the same square but capture enemy pieces en route - very far from a "null move"! MD versions of the Marshal and Cardinal have only their long-range moves affected.
The piece which inspired my interest in multiple displacement is the Xiang Qi Elephant. The MD version of this gains two powers - to access a destination blocked by an enemy, and to capture on files and ranks that it cannot occupy. This piece can simply be termed the MD Elephant, as it still has most affinity of any MD piece to the Chaturanga Elephant (also known as Alfil - ed.). By analogy there is a piece that could be called the MD Dabbaba, again blocked by its own side but able to capture on over half the board - surprising when it can occupy only a quarter of it.