I believe this to be a prey response similar to a possum playing dead. Birds have stiff lungs that are supplied air via air sacs, so it shouldn't restrict breathing much, if any. Still, I have noted open mouth breathing when birds are on their backs, so I tend to restrict use of their tendency to go limp like this to things like treating SLM or trimming toenails. Holding a bird by the legs also produces a similar trance like state where they can be hung for slaughter or (in the case of a flighty bird) moved easily. Be careful of the possibility of regurgitation and aspiration.
Sources: personal experience with my flock (~200 birds to date) plus research into the anatomy of chickens.
I believe this to be a prey response similar to a possum playing dead. Birds have stiff lungs that are supplied air via air sacs, so it shouldn't restrict breathing much, if any. Still, I have noted open mouth breathing when birds are on their backs, so I tend to restrict use of their tendency to go limp like this to things like treating SLM or trimming toenails. Holding a bird by the legs also produces a similar trance like state where they can be hung for slaughter or (in the case of a flighty bird) moved easily. Be careful of the possibility of regurgitation and aspiration.
Sources: personal experience with my flock (~200 birds to date) plus research into the anatomy of chickens.