Why can you reduce after a null move?

[Henk]

I don't understand why you can reduce after a null move. I mean in same path to the leaves of course [By the way within a tactical combination one should even extend. But how can it know if it is within a tactical combination]
Don't like it if it misses a fork or a checkmate. Don't come with statistical or pragmatic arguments please.

[hgm]

With a true move you can usually quite easily delay the inevitable, in order to push it beyond the horizon. E.g. you do an extra equal trade, or you attack a valuable piece of the opponent that he then has to wihdraw, without changing anything essential on the position. But burning two ply nevertheless.

A null move can never be used to delay anything; the opponent can start doing his worst immediately. So if there is trouble that you cannot see, a deeper search of your 'best' real move (i.e. the one that delays the problem) is very unlikely to see it either.

[Henk]

Say the reduced path leads to a stand path position with a fork. But fork is not counted so you get a wrong score. But if it had not reduced then it would have seen that it lost a piece. Just a one ply difference. Or is this the unlikely case.

O wait I did not read your answer carefully enough. It is about null move can't delay. But is a quiet move not delaying as well. So that means you can reduce all harmless quiet moves as well.

[hgm]

In principle you could. Except, how would you know whether a move was quiet or not? Chess is a very dynamic game, moves can have all kinds of non-obvious side effects. But you can be certain the null move doesn't have any.

[Henk]

By using the same trick as LMR perhaps. Scan and research. Better use smal steps perhaps. So i can reduce with a small step if effect of a move is expected to be low.

O wait. Scan may be expensive as well. Unless I scan with a reduction of 4 or more. Doubt if that would be worthwhile.

[Henk]

Get the impression that it does not play better with null move reduction against stockfish. Maybe add a margin to compensate for the lost ply. Don't know.