[Al Skierkiewicz]

Just a quick response here and then I will be quiet for a couple of days again while traveling.

R77. H. Shutoff valves which relieve downstream pressure to atmosphere when closed (may also be known as 3-way or 3-way exhausting valves).

While these are now legal, they have not been allowed or legal in the past. Each manufacturer has a different name for them, (thank you marketing people) they are what I believe the discussion is about.
The Bimba part linked above is one of the devices that I believe influenced the addition of this rule. It is a 3-way valve in that air at one port is directed to another port to move the cylinder. When air is removed, the pressure in the cylinder is directed to atmosphere at the third port. The Bimba spec sheet gives an example of a method to move a double acting cylinder with a two way control valve and two of the linked valves.
Check valves have also been included in R77.G if and only if R89 can be satisfied. The Bimba device will exhaust all stored air when system pressure is removed.