The BDS Website

BDS

British Chess Magazine, 1977

#2
U-Chess

1.B-N5

1...P-B3 2.R-N4
1...P-B4 2.RxB
1...K-N2 2.B-R6

The checking key gives a flight at b7 and unpins the bPf7. It unpins because, after the key, the threatened RxK would be illegal because ambiguous. Each move of the bP interferes with one of the black bishops allowing White to release the ambiguity of RxK my moving the wRc4 to shut-off the other black bishop. If Black takes his flight, then White mates with the orthodox 2.B-R6.