It's relatively simple: lockdowns are effective to the extent that they reduce contacts between people. In countries that actually implemented strict lockdowns that substantially reduced contacts between people, transmission fell, R went below 1, and the epidemic receded.

The most effective strategy for countries that have large numbers of cases is to go into a strict lockdown, in order to bring case numbers down as quickly as possible. Then they can reopen to a much larger extent than they would otherwise be able to. China and several other countries have successfully done this.