It's all described in the readme, but generally online schema change tools work by creating a new table, copying the data from the old table over, somehow keepi

It's all described in the readme, but generally online schema change tools work by creating a new table, copying the data from the old table over, somehow keeping track of new writes to the old table, and then syncing those over. At the end the tables are swapped. With gh-ost you can pause the writes to the new table.