The R factor in a pandemic determines ability of the virus to spread. R > 1 means that each infected person infects more than 1 person. The higher the R number, the higher the case increase. Lockdowns don't eliminate the virus, but they can reduce the R factor because of the reduction in contact between people. When they talk about a 20% reduction, you can see the impact on the case increase when R goes from 1.1 to 0.9. And a further 5% reduction would have an even greater impact on reducing the case growth.