These joints are absolutely amazing. Not only do they seal gas pressure, they move with minimal friction, independent of axial pressure on the joint. Any conventional joint design would “lock” due to pressure. Even the 4.5 psi pressure in an EVA suit would put over a half ton of axial pressure on a torso joint. Providing low friction movement under these loads is a much bigger challenge than sealing. Used on atmospheric pressure diving suits, joints are certified for use at 1000ft. That produces about 125,000lb axial compressive force on a torso joint.
The key feature which allows rotation under pressure is a ball bearing race, visible as item 134 in the patent drawing below and mentioned in https://www.americanscientist.org/article/the-past-and-future-space-suit
There are numerous seal cross section which are proven performers. “O-rings” are the most common, and is the design used in the patent application below. "X-rings" are similar to O-rings" but have an "X" cross section so they have4 sealing contacts instead of 2.
The joints have a double seal, with no load on the second “back-up” seal. So if the primary seal fails, the secondary seal is “virgin”. Detailed patent drawing available at https://patents.google.com/patent/US6725464B2/en
