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.
You can find more information about endgame studies at EG.