Read the board, again looking for alignments of enemy pieces—and especially enemy pieces aligned with nothing between them. White has a few pieces so arranged: his bishop and queen are on the same diagonal, as are his bishop and rook. With Be6 Black can run a bishop through the latter two pieces. In itself this doesn't achieve anything, but with a substitution it might. So imagine exchanges and consider whether they would improve your prospects. Here Black’s knight on e5 can take White’s bishop on c4. This requires White to recapture with QxN. Now we have the kernel of a productive skewer; we have created again the classic queen-before-rook pattern. All that’s left for Black is to get a bishop safely onto the same diagonal. He plays Be6, which in effect attacks two pieces: the queen and the rook behind it. White retreats the queen to e2 so it can guard the rook, and then Black wins the exchange.