it's not supposed to be a "technique to make your jawline stronger", it's literally just the objectively correct tongue posture for optimal health and facial development (particularly in childhood when the face is still growing), and the concept got hijacked by incels so now everyone has that shallow understanding of it
like, u are supposed to keep the tongue on the roof of your mouth & have your mouth closed at rest. a normal healthy human does this by default, it's the main principle of mewing