White's bishop and queen are characteristically arranged for a discovered attack, with Black’s loose queen making a fine target on d4. Everything is prepared except a good threat for the bishop to make when it vacates the d-file. It would like to give check, naturally, but its lines to the king from b5 and g6 are blocked. What to do? When your goal is to clear a line, consider attacking pieces that are guarded by the pieces you need to move. Here the lines that White's bishop needs to give check are blocked by Black’s d7 bishop and f7 pawn. If White can capture something that one of those pieces protects, then the piece will have to move to recapture, a line will be cleared, and perhaps White can win Black’s queen with a discovery. As it happens, both of Black's blockers protect the pawn on e6, and White can take it with Rxe6+. Now if Black recaptures by any means, he opens a diagonal leading to his king, and White has a discovery that does win the Black queen on the next move—either Bb5 or Bg6. Obviously Black can capture the bishop easily either way, but you don't care; you just want to make him spend a turn that way so you can take his queen.

In practice, seeing all this just gives White a way to take a Black pawn for free (and improve his position a bit), since Black would of course rather lose the pawn of e6 than his queen; if he is attentive—and you should assume he will be—he will reply to the capture by moving his king to f8, not by recapturing and setting himself up for the discovery. Many of the positions we are studying work that way. In practice they yield a payoff, but not necessarily the first and best one you see; for your opponent may see the coming disaster and choose to make some lesser sacrifice to avoid it.