Figure 2.4.5.4[White to move]

When the queens are faced off against each other like this, exchanging them is an option both sides have to consider at every turn. A capture by either player will require an immediate recapture, which may open lines, leave loose pieces elsewhere, etc. Here White imagines QxQ and Black’s compulsory recapture NxQ. This simple sequence leaves behind loose Black pieces on c5 and f5 (whereas there were no loose Black pieces before), and a clear path for White’s rook from e1 to e5 (whereas there were two pieces in its way at the outset). Now the rook fork Re5 wins a piece. This position is a good illustration of how a single exchange can radically alter the tactical opportunities on the board. The solution is easy enough to spot if you understand the significance of loose pieces—and probably impossible if you don't.