Tandemaus' ability Sequence is a unique and unobtainable ability in competitive play exclusive to this Pokémon and not available via regular gameplay modes. The concept of Sequence anecdotally implies it might involve some kind of pattern-recognition or turn-sequencing effect, but there is no actual implementation of such an ability within the existing gameplay framework.
Since Sequence is noted as an unobtainable ability, it doesn't have defined effects in the data files or competitive environments. Consequently, it doesn't impact gameplay mechanics, unlike other abilities.