Thursday Tactic

Enjoying the warm summer evening with my bride, sangria, and a cheese plate, I opened the Chess Informant Solver's Kit and chose a random set of problems. On the first, I failed until the third try. The correct answer, once discovered, is remarkably simple and potent.

White to move

6k1/5ppp/pB6/3br3/Pp6/5QPq/1P3P1P/2R3K1 w

From Lutskovsky -- Gridnev, correspondence 1976.