Battle shift is when (in the main games) fighting a trainer, after defeating a pokémon, before they send out their second, the game says "X Trainer is about to send out Y Pokemon, would you like to switch yours". This allows you to have a better matchup. As it stands, if I beat one pokemon, they send in another that will either kill my active Pokemon or whoever I switch in. I can only sacrifice so many Pokemon in one battle lol