Do you see an apparent opportunity for White? A pawn in the center always is an important focus of attention; here Black’s pawn on e4 is attacked twice by White and guarded once by Black, making it possible for White to play Nxe4 with impunity. Or so it seems. But before making even a simple capture, assess the board as it would appear afterwards. Once White plays Nxe4, what are Black’s options? What does he threaten? The important answers are the attacks by Black’s queen on White’s queen and by Black’s knight on White’s knight. The fortunes of those attacked pieces are linked. Black can play QxQ+; and then, after the recapture f2xQ, he plays NxN, having loosened White’s knight. Another way to see the point is that even at the outset of the position White’s queen is under attack by Black’s queen. This makes the White queen a poor defender of anything else, as it can be eliminated by Black in a single stroke. The simple lesson: don’t send your pieces on adventures with protection only from fellow pieces that currently are attacked. (Actually there is an even better reply for Black here that we will revisit in the chapter on distracting the guard. Perhaps you can see it already...)
Incidentally, notice an idea here for White that nearly saves the day—but not quite. After 1. Nxe4, QxQ, White almost can play NxN+, pushing the same knight onward not only with a capture of an enemy piece but also with a fork of Black’s king and rook that requires Black to spend a move recapturing (the priority of check). (And if Black plays Ke7, White next plays NxR+ and checks yet again.) The only reason this doesn’t work is that Black’s prior move—QxQ—is itself a check that requires White to spend a move replying f2xQ. But all this would give White a fine out if his king were positioned a little differently. Another old lesson repeats: don’t take for granted that your opponent will reply to your captures by recapturing; he may be able to make other captures or threats of his own first. The beauty of working with checks is that they avoid those risks.