The way most people treat their pools is to throw some pucks in it. This works for a while, but without anyone actively monitoring FC and CYA levels, eventually FC drops too low for the CYA. This could be because not enough FC is getting released in general or because the CYA is getting too high.
To extend how long people can do this without exchanging the water, the people maintaining their pools this way do a weekly "shock" - bringing the FC levels up extremely high. The hope is that this will kill off any algae starting to grow, because eventually with the CYA getting so high, algae will start to grow. And it might kill off that growing algae, for a while.
Eventually the CYA builds up so much that the "shock" doesn't even bring your FC up high enough - or if it does, it doesn't stay high enough long enough for the elevated chlorine levels to kill any algae growing. The growing algae and sun bring the FC back down below where it's effective, and the pool goes green. Then they're told they have "too high of total dissolved solids" or "chlorine lock" and have to drain/refill the pool. This brings their CYA levels down, and the cycle starts all over again.
Instead, we maintain our FC where it should be. Algae can't start growing. So no need for a periodic "shock." If we slip, and our FC levels do drop too low and algae starts, we use the SLAM process as a careful, metered way of keeping an elevated FC that's high enough to kill algae without being so high that it damages pool surfaces and equipment. But people who are careful with their routine never have this happen, and so never need to raise FC past maintenance levels.
