Feather mechanism
The BQ stands at the junction of the lines of a WB and a WR while the BK stands poised to move onto the continuation of the WB and WR lines beyond the BQ’s square, one of which in each solution needs to move through the BQ’s square. In moving aside to allow this the BQ creates a guard from the other white piece on the square to which the BK must move. Consequently the BQ must clear the line for the WB by capturing (or interfering with) the WR and vice versa.
Feather, Christopher John
Schach-Echo, 1976
h#2 2.1... (5+9)
1.Qxb6
Rc1
2.Kd4
Qxd2#
1.Qxc6
Bg1
2.Kc4
Qb3#
Feather, Christopher John
Moultings 5, 1991
h#2 2.1... (7+11)
1.Qxg8
Rf3
2.Ke6
Re2#
1.Qxf8
Bc4
2.Kf6
Bc3#