From: http://www.saunalahti.fi/~stniekat/pccc/8themes.htm
In a position in the main line of a win or draw study
where an unprotected white or black piece A is directly attacked, White or
Black instantaneously (right on the following move) places another piece B (of
the same colour as A) en-prise (again unprotected and directly attacked). This
thematic move resulting in the two white or the two black pieces A & B
hanging, must be a quiet one, i.e. not a check, nor a capture. A and B may be
any pieces except pawns.
The studies were extracted from the Harold van der Heijden database using a CQL script, wcct8.cql, originally written by Emil Vlasák.
You can find more information about endgame studies at EG.