Double Switch
Two substitutions at once so the new pitcher bats later.
How it actually works. The manager brings in a reliever and a position player at the same time and assigns them to each other's places in the order. The reliever takes the lineup spot of the fielder coming out, which is chosen to be a spot that just batted, and the new position player takes the pitcher's old spot, which is due up soon. Nothing about who plays where on the field changes for the position players. Only the order changes.
Why it is done, and when it is wrong. The whole point is squeezing more innings out of a reliever. If the pitcher's spot is due to lead off the next inning, a reliever brought in normally throws two thirds of an inning and then gets hit for. Switch him into the spot that just made the last out and he has eight hitters to get through before anybody has to bat for him, which is often two full innings of work. It is a purely National League skill in origin and it became close to extinct when the designated hitter arrived in both leagues in 2022, surviving only in games where the designated hitter has been forfeited, which happens when a club moves its designated hitter into the field.
What people get wrong about it. The trap is defensive. The spot you are switching out is chosen by the batting order, not by who you want on the field, so a manager can easily find he has taken his best defender out of a one run game to save a reliever forty pitches. The other trap is that it is announced to the umpire and it is irreversible.
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