Flatlock stitching lies flush against the skin rather than raising a ridge, which is the specific detail that prevents mat burn during BJJ scrambles — regular overlock seams are what cause the chafing rash guards exist to prevent in the first place.
BJJ and wrestling academies ordering matching no-gi training or competition rash guards are the primary buyers, and the UV50+ rating is an added benefit for teams that also train or compete outdoors.
Rinse promptly after training rather than leaving a sweat-soaked rash guard balled up in a gym bag, since prolonged dampness against spandex accelerates fabric breakdown faster than the washing itself.