This seems like a commonly overlooked factor and I believe releasing the tension through the myofascial release massage and then stretching exercises eg these ones does help with the Mewing work. Other good adjuncts are use of epsom salts, plus other practices to assist nitric oxide release and waste removal (eg: cup massage, lymphatic massage, energy devices, exercise, foods and supplements). Hydrogen water has helped get my hydration levels up to become overall more pliable. Despite all these practices I was only able to finally release the chronic tension in my eyebrows with my PEMF device. I think the chronic tension damages the tissues, then pressure, massage helps gradually fix the damaged collagen and encourages blood flow to the area and assist the flow of lymph. Getting the cranial bones in to better position is going to passively help lymph flow. After doing most of these practices for several years, it's hard to pin-point how it's helped in relation to the Mewing work, but I do see and feel I've made the skin and muscles healthier which at least looks better and feels overall more comfortable.

@greensmoothies Especially when people recommend hard mewing or anything similar that requires unnatural force, I think they end up adding even more facial tension; I know when I do hard mewing, my whole natural resting chain gets disturbed. Facial tension is probably largely unknown to most people unless it's so bad that it's obvious or if they're constantly looking in a mirror, which is how I became aware of it. One might assume that people with forward grown faces don't have much tension because their face usually looks very relaxed, but giving any credence to any possible causation would probably require knowing how widespread of a problem this is, since only really bad scrunching, etc. are obvious cases.

The blog owner mentions being able to press on his brow bone with a knuckle or baseball without any pain, which I always thought was hyperbolic, until I tried knuckling my knees, forearm bones (being underweight makes them all quite similar) among other places and didn't feel any pain. The amount of pain from knuckling my brow has noticeably decreased, but there's still some there. Unless there's something different about the bone in your forehead, perhaps it just shows how much tension has accumulated. Any possible facial changes will probably take months of doing this, but I'll keep doing it.