Zwischenzug
Borisenkov-Mezenev (diagram, Black to play), went 1... f2, threatening to queen, which White countered with 2. Rg8, intending 3. Rf8+. But White resigned after the zwischenzug 2... Bb1! which allows Black to queen (3. Kb1 f1Q+ or 3. Rf8+ Bf5).