The addition of 5 - 10 p.p.m. of free chlorine in response to a particular need. Alternative non-chlorine shocks are a matter of adding the necessary sanitizer to meet the contaminant burn out. This is the only time that the water is truely 'sanitized'. The residual treatments (i.e. tablets, ozone, etc.) work merely to keep the water from going green between shocks. The amount needed, is based on the demand. If a chlorine based pool, without reduction, has 0.0 p.p.m. chlorine, it takes twice as much as what is needed for the same pool with 2.0 p.p.m. to reach the same level! Presence of algae can double that, etc. Shock is needed when the free chlorine is exceeded by the total chlorine, signifying combined chlorine (chloramines), needing to be oxidized.