Alan;

This is the gist of what Mees and James report from experiments many many years ago and what Patrick has excerpted the figure from. They found that any sulfite in the presence of Metol or HQ will speed up their individual reactions due to the fact that Sulfite reacts with the oxidized forms of these developers.

On top of that HQ and Metol are superadditive, and so you have a multiplicity of effects going on.

PE