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Black has just blundered with 32 ... xb3?, totally missing White's threat.

White would like to play f6-f8#, a rook and bishop mate. Only the Black g7-knight prevents this, by <BLOCKING> the diagonal h6-f8 of the White h6-bishop.

So the Black g7-knight is <PINNED> to the h6-f8 diagonal. Then which Black piece defends the e8-square? None! The e8-square is undefended because the Black g7-knight cannot go there, since that would allow the aforementioned rook and bishop mate.

<The defensive power of a pinned piece is merely illusory, Nimzowitsch said.> The Black g7-knight, pinned to the h6-f8 diagonal, only pretends to defend the e8-square.

Note that the Black g7-knight has two ways of meeting the threat of f6-f8#. The knight can stay on g7 where it <BLOCKS> the h6-f8 diagonal. -Or- it can move to the e6-square, from where it <DEFENDS> the f8-mating focal point. So the Black g7-knight, in order to meet the rook and bishop mate threat against f8, must stay on g7 or go to e6. But there is no way the Black g7-knight can meet the mating threat if it goes to e8, so it -cannot- go to e8.