(Taken from https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html.)
pt-online-schema-change works by creating an empty copy of the table to alter, modifying it as desired, and then copying rows from the original table into the new table. When the copy is complete, it moves away the original table and replaces it with the new one. By default, it also drops the original table.
The data copy process is performed in small chunks of data, which are varied to attempt to make them execute in a specific amount of time. Any modifications to data in the original tables during the copy will be reflected in the new table, because the tool creates triggers on the original table to update the corresponding rows ...
