Bishop X2 is pinned by R at X3. A pinned piece obstructs but is hampered in its capacity of capture or guarding. Therefore, White with impunity captures the P with Q. This Pawn is guarded by the Bishop only in appearance, not really. The pinned Bishop is now twice attacked, by Rook and Queen, and Black is unable to guard it the second time, except by Q-B3, which is futile on account of QxQ.