A Power Sub is the manual action of dropping a bench player who did not yet play in the starting line-up. That type of substitution is useful when the player in your starting line-up underperformed (low or negative score) or did not play at all.
A “late sub” or “late swap” can be a feature of a premium package. Everyone has auto subs, but the power swap gives you more control. When the gameweek deadline has passed, you can’t change anything anymore. The only restriction is that a benched player can only be power subbed when he has not yet played. If he already played, he remains on the bench.