Quoting from a previous message,
"At 1:1:100, the total concentration of pyrocatechol will be 0.05g/liter, plus 0.0002g Phenidone. This is less than a fifth of the lowest concentration in any other developer formula?"
The very small amount of reducing elements in a liter of working solution is one of the reasons for the outstanding sharpness of the Pyrocat-HD formula. As a rule develolpers with very small amounts of reducing elements tend to give sharper negatives than those with larger amounts. D76 1:1, for example, is visibly sharper than D76 straight.
Regarding the amount of Phenidone, I see that you corrected in another message the above amount. The amount is still very small, but phenidone is strongly super-additive with pyrocatechin, and it also has strong regenerative qualities. And at the given amount of 0.002g per liter of working solution it plays a very big role in development. Metol, even when substituted in the PyrocatHD formla at 10X the amount of phenidone, impacts develoment less than phenidone.