 This not only threatens 31...Nxe6, but also enables Black to meet 31.Rxb8 with 31...Qd1+ 32.Kh2 Qh5+ 33.Kg1 Qd1+, drawing by perpetual check.The perpetual check is based on White's weak back rank combined with his slightly compromised king position (no h-pawn).Note how pieces that are well placed for attacking purposes may be misplaced for defensive purposes.