So let me translate it back to the chemical from electrical: the chlorinated cyanurate will release some hypochlorous acid as needed, so as it gets spent there will be more released. OK.
Why then do we keep talking of minimum FC needed when stabilized and if below, FC being ineffective?
It is not that below a certain FC/CYA ratio which is proportional to the active chlorine (hypochlorous acid) level that it is completely ineffective. It is less and less effective in that the concentration is below the level at which the rate of chlorine killing of algae is faster than algae's growth rate. The FC/CYA ratio level that is in the Chlorine / CYA Chart in the Pool School is set somewhat above this threshold where the chlorine kill rate of algae equals algae's best case growth rate aka generation time or the time it takes algae to double in population. Under ideal conditions with plenty of algae nutrients, ideal levels of sunshine, and ideal temperature, algae doubles in population every 3 to 8 hours. The concentration of active chlorine needs to be at a level where it is able to kill half the algae faster than this timeframe, otherwise algae grows faster than chlorine can kill it so will have net growth, increased chlorine consumption, and eventually visible algae (or dull/cloudy water first, then green).
I think you are stuck on the fact that if there is chlorine in reserve and it is able to be released quickly, then why does the active chlorine level matter. The reason is that the chlorine bound to CYA does not kill algae and it can be instantly released but will not affect the kill rate since that is solely dependent on the active unbound chlorine concentration. An analogy might be helpful. Consider soldiers fighting an enemy and where only the soldiers on the front-line have weapons to kill the enemy. The rate of killing the enemy has only to do with the number of soldiers on the front line. It doesn't matter how many soldiers you have in reserve who can replace any killed front-line soldier even if such replacement is instantaneous. The reserve just tells you how long you can continue to fight, not the RATE at which you are killing the enemy.
Chlorine bound to CYA (and to some extent hypochlorite ion) is like the soldiers in reserve. Hypochlorous acid is like the front-line soldiers with weapons.
Also as was noted in the previous post, the FC test measures all the chlorine because as the hypochlorous acid is used oxidizing the dye or reacting with FAS reagent, more is released from CYA and this is faster than the time of the test. About half of the FC is released from CYA every 0.5 seconds if the hypochlorous acid were to instantly disappear. So what you are really measuring in the chlorine tests is mostly the chlorine reserve or capacity, not its rate of kill or effectiveness. The FC/CYA ratio is a much better proxy for determining the chlorine effectiveness or kill rate against algae, bacteria, viruses, etc. though obviously actual kill rates vary depending on the species. Generally speaking bacteria are easiest to kill, then viruses, then algae, and then protozoan oocysts and bacterial spores and biofilms.