Like other Tai Ji Chuan and internal skills, we need to pay attention to them, as do general qigong and internal skills. In fact, it doesn't need to be achieved too deliberately. If you relax your tongue, it will naturally touch your upper jaw, that's all. You don't need to think deliberately when practicing, just concentrate on playing. (Unless you are used to loving with your mouth half open, you need to pay attention, hehe)