Figure 2.2.3.6[White to move]

Black has just played e7-e5. The pawn he has left on e5 has no protection, so White appears to be able to take it for free with his knight. Again, however, notice that the move would leave the knight loose. Notice also that the Black queen can give check with Qa5 and at the same time can attack the fifth rank. So White must not leave any pieces loose there; Nxe5 would lose the knight to Black's queen fork a move later.