Automatic substitutions or “auto subs” replace players in your starting lineup who didn’t play with players on your bench.
Your starters are always prone to injuries before or not playing because of a tactical decision by their coach. Auto subs provide cover for these unforseen circumstances.
Auto subs take place at the end of the gameweek when all matches have been played. Your lineup still needs to be in accordance with the formation rules. In fantasy football a goalkeeper can only be auto subbed with another goalkeeper. And an auto sub can only go through if your starting lineup still has enough players of the same position (eg. at least one forward or three defenders).
Auto subs are carried out based on the bench priority you assign to each of your players. The player on position “1” will be auto subbed first, and so on.
An auto sub is not to be confused with a power sub.