The Special Defense stat of the pokemon using Amnesia will increase by 2 steps (termed BTLs in the Gameboy description) on the Stat growth scale.
Because of the uniformity of the increases of the value, this increase translates into an increase equivalent to the original value of the Special Defense stat each time this move is used. In more definite terms, starting with no modifiers, the 1st use of the move will make the Special Defense stat 2 times the original value, the 2nd will make it 3 times, and the 3rd will make it 4 times the original value.
Amnesia will always fail if it has already been used 3 times by the same pokemon, without a move that lowers Special Defense being used on or against the pokemon using Amnesia. This is due to the fact that a stat may only be raised 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 increases can be applied to a stat. If another move increases the Special Defense stat of the pokemon using Amnesia, the number of steps that it increases the Special Defense stat by will count toward the 6 step limit. This will not reduce the number of times that Amnesia can be used unless the increase is by at least 2 steps. The increase will still max at 6 steps, so Amnesia will only provide a 1 step bonus if there are already 5 steps of increase in effect.
