1.e4 threat 2.Kd7+ Kd4 3.Kc6 or 2…Kb5 3.Kd6;
1...Sxc7 2.Rd3 threat 3.Kd8;
1...Bxc7 2.Ba4 threat 3.Ke8;
1...Rf5 2.exf5 threat 3.Kxf7;
(1...Sb8 2.cxb8 any+).
The successive royal battery openings of the threat are countered by
self-pinning defences. The resulting mutual obstructions between the
knight and bishop are exploited by White’s square vacating replies.
Jacob Hoover: In the diagram, 1...Rxc4 is a strong defense, which
suggests that 1.e4 is the key. The pair of thematic defenses involves
mutual obstruction.