"Shock" means to raise the chlorine level past the breakpoint which forces a chemical reaction that gets rid of combined chlorine (CC). This point varies depending on your CYA level - the higher the CYA, the higher level of chlorine needed to reach the breakpoint. Ben's best guess chart will give you an idea of CYA vs. chlorine.

There are products labeled as "shock" but it is just concentrated chlorine to boost your Cl level up quickly. If you add enough bleach (look for the BleachCalc program from mwsmith2), you will reach the shock level too.

Peter