The white mating piece arrives (on the mating move) on a square that has just been vacated by a black piece.
See also: Umnov move; Consecutive Umnov; Closed chain of Umnov; Delayed Umnov; FML.
Avner, Uri
J. & M. Figueiredo MT, 2011
1st Prize
a) 1.Sc3
Sa4+
2.Sd5+
Sc3#
b) 1.Sd6
Sb7+
2.Sf5+
Sd6#