String Shot will lower the opponent's Speed by 1 step (termed BTLs in the Gameboy description) on the Stat growth scale.
String Shot will always fail if it has already been used 6 times on the same pokemon, without a move that raises Speed being used by or against the pokemon affected by String Shot. This is due to the fact that a stat may only be lowered by a maximum of 6 steps, until it is reset by the use of Haze or the pokemon is switched out normally. The maximum is the limit of how many decreases can be applied to a stat. If another move decreases the Speed stat of the pokemon that String Shot is used against, the number of steps that it decreases the Speed stat by will count toward the 6 step limit, and reduce the number of times that String Shot can be used appropriately.
String Shot is affected by Brightpowder when the opposing pokémon is equipped with it.