Yes there is a piece of plastic that pushes inwards when you attach the tap. When empty you recover the plug and put it back into the rubber bung when you refill.
from that i have seen of them they are the same as the gray bungs just with a valve in it. so the valve should push in to the keg or you would not be able to get the bung back out the keg. but if you priming the keg with the right amount(16 grams) of sugar in the first place you shouldnt need a pressure valve.
i use the cheap 3 part bung but all my kegs have a built in tap i also use a vintage 2000 party tap
the grey bungs have a single piece bit that pushes in the keg with the tap going in.
black bungs with prv's have a 2 part bit the top bit u pull up and twist to get off and the bottom bit gets pushed in as per tye grey bung arrangement.
