When a Pokémon Move has the description that it "drastically" raises one of your Stats. It raises it by three stages.
An Example: Tail Glow
Tail Glow increases the user's Special Attack by three stages from Generation 5 onwards.
Tail Glow's Pokédex entry is:
The user stares at flashing lights to focus its mind, drastically raising its Sp. Atk stat.
Hope I helped :)