Interference

The first diagram shows a position from the game Janowski-Schallop, Nuremburg 1896. White has active pieces but seems to be losing the initiative because of the counterattack on his rook. He solves that problem with a surprise move:

1.Bd5!
An interference sacrifice, closing off the defence of c6 by the Black queen. If Black plays 1...Qh3 then White can simply win a rook with 2.Bxc6+.
1...exd5 2.Qxc6+ Kd8 3.Qxa8+ Kd7 4.Qb7+ Ke6
Other moves are obviously no better. Black is continuing out of pure inertia.
5.Qc6+ Bd6 6.Bf4!
Black resigned. He is mated after 6...Qxh1+ 7.Kd2 Qxa1 8.Qxd6+ Kf5 9.Qe5+ Kg6 10.Qg5.

The second diagram shows a position from the game Scoones-Gregg, Portland 1976. My opponent has sacrificed a piece for some counterplay against my king, which is more annoying than it looks because of my passive bishop on g3. After studying the position for a short time I saw an opportunity to break Black's resistance by sacrificing the bishop in unusual fashion:

1.Be5! dxe5
If 1...Bxe5 then 2.Qf8+ Kd7 3.Bf5 mate. The strongest line of resistance is 1...Qa3+ 2.Kd1 dxe5 3.Qxg7 (not 3.Qg8+? Qf8) 3...Bxf3 4.gxf3 Qd6 but after 5.Ke2 the win is not in any doubt.
2.Qg8+ Kd7 3.Qxg7+ Kd6 4.Qxh6+ Kc5 5.Qxg5
Black's position is hopeless and he resigned after a few more moves.

Janowski's combination must be valued more highly than my mundane effort, for two main reasons:

1. In Janowski's game, the sacrifice is the only way to win or even avoid serious disadvantage;
2. In my game, the sacrifice is effective but not strictly necessary. White is already a piece ahead and has an alternative winning line that starts with 1.Qg8+ Kd7 2.Bf5+.

I do not believe I had seen the Janowski combination when my game was played. But I know I had seen something like it. One must study the classics!