Bulbasaur at lv153 that knows explosion?
Is explosion the only move it knows?
No, Explosion is 3 our of four of it's moves.
The other move is whatever it's third move was. (eg; Mega punch)
Because 3 is safe from the corruption because it's a multiple of 3, meaning that the 3rd move is unaffected.
The same applies for any other multiple of three.
(Please correct me if I am wrong)