Edwal TST is another contrast control developer.
It has a unique way of working. Part A has sodium
sulfite, sodium hydroxide, hydroquinone, and I believe
phenidone although the MSDS does not so state. Part
B contains sodium metabisulfite and ?.

From the composition of the two parts I've concluded
that contrast control derives from the adjustment of the
developer's ph. The addition of the acidic part B. will reduce
the ph and inactivate the hydroquinone. Clever What? Dan