It strengthens attacks, pwns enemies, and makes you look badass. if you repeat ATA, as ATATATATATA!!!! then you're probably throwing rapid barrages of punches at some poor motherfucker.
It also variates into "WATA!!!" "ATOU!" and various other similar shouting.
He shot a Hundred rapid Fists at the man, and each punch was in sync of his shouts: ATATATATA