Examples

White can deliver double knight mate with Nf7#.
The knight on e7 guards the escape square g8.

White can deliver double knight mate with Nxf4#.
The knight on f4 also guards the escape square g2, and the knight on f6 is guarding g4.
The rook on d3 helps by pinning the pawn on g3, preventing the capture of the checkmating knight.

Pattern matching

This happens usually with the king in the corner or on the edge of the board.

Related patterns

In Smothered mate, a knight checkmates the king who is blocked in by its own pieces.
In dragonfly mate, a knight checkmates the king, and a bishop guards prevents the escape.