notyetagm: 21 ... c7 threatens the diabolical <discovered deflection> 22 ... xd4!. The tactical point is that the capture of the White d4-knight and White's subsequent recapture by the White d2-queen (22 ... x! 23 x?) removes not one but two(!) defenders from the c2-square, by <destroying> the d4-knight and <deflecting> the d2-queen.

Black would then have a classical back rank mate in three with 23 ... xc2+ 24 a1 c1+! 25 x x#. Hence White could in fact not recapture on d4 and would thus lose a piece for nothing.