The point is that the chlorine that is bound to CYA doesn't react with organics. That's why we have to increase the FC level according to the
FC/CYA Levels, keeping the FC-CYA ratio constant, so that the active chlorine level (HOCl) is high enough to kill organics. As soon as the HOCl starts doing its job and its concentration in the water gets down, more chlorine gets released from CYA (following the equilibrium equations) to form fresh HOCl that keeps the sanitation process going.
The chlorine that is bound to CYA (and that is most of it) doesn't react with organics - that's why a pool with sky high CYA and standard FC levels (not following the FC-CYA chart) turns green. But in the same way, this chlorine that's bound to CYA is also not aggressive to swimmers or pool equipment - you can safely add more chlorine following the FC-CYA chart until you have enough HOCl in the water to keep it clean and safe.
The extra chlorine that's bound to CYA is a kind of reservoir chlorine that is inactive while bound, but once HOCl gets used up, it gets released fairly quickly. Like that you can have enough chlorine in the water to eventually react with all organics or survive a day of high UV load, without all of the chlorine being "active" at the same time, i.e. the chlorine won't be harsh.
In fact, the chlorine bound to CYA gets released fast enough that it shows up as Free Chlorine in the FAS-DPD test (and other chlorine tests). As the titrating reagent "deactivates" the HOCl (and OCl
-), more chlorine gets released from CYA, until all of it is released and showing as Free Chlorine - in that way the titrating reagent is a kind of "super contaminant".
That's the big misunderstanding - the "Reservoir Chlorine" shows as "Free Chlorine", but it's actually not "free". Unfortunately, there is no easy test to directly measure HOCl. The best workaround is to measure FC
and CYA, and calculate HOCl based on the chemical equilibrium equations - that's basically the theoretical background of the FC-CYA chart.
Sorry for that big detour around the original question, but I thought it was important to make that point after RDspaguy brought it up.