Question

NaHSO3 is an ambient nucleophile but how?

Answer 1

You have some terminology confused.

I believe you're referring to `"ambident nucleophiles"`, whose negative charge is delocalized by perhaps the most important thing in organic chemistry: resonance!

Bisulfite would normally look like this,

But a hybridized (and thus more accurate) structure would look like this,

Note: `-` denotes a partial negative charge `(delta^-)`.

Hence, due to its `pi` system about the sulfur atom, its charge is delocalized.