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The target is forced to move last in the turn Quash is used, even if the move the target uses has positive priority, such as Mach Punch. If the target has already moved before the user, or if all other Pokemon have already moved, then Quash will fail.
A very unique move introduced this generation. This non-damaging move will make the target always goes last, failing if the target has already moved, and thus only works on the turn it was used. It doesn't matter what move the target may be using; it will go last regardless. Obviously, it will never make a priority move go last, because of its own normal priority. But it can make a move with normal priority move after Trick Room, the slowest move, priority wise, in the game. If, for some reason, you need something like that, for example in doubles.
After You forces the target to move first rather than last.