Swagger is a move introduced in Generation II that increases the Attack stat two times of the opponent and confuses it, the Confusion stat depends on the Attack and Defense stat of the Pokémon that has this stat. This move is very strategic, because the confusion problem damages a lot, though a good combo will be using Screech to double damage.
Using this move on a Pokémon with a Persim Berry or Own Tempo, only increases Attack stat. Using it on an opponent is a disadvantage, but using it in a Double Battle to your teammate will be an advantage. Also if it's use with Psych Up to increase both Pokémon Attack or in a Single Battle can be used to copy the foe Attack stat without getting confused.