It's a torture device that appears to prevent the cat from accessing
its head with its paws and adds a possibly significant weight (a
percent or so) to its neck.

I noticed lots of satisfied customers. I also noted the complete absence of
unsatisfied customers.

I'll let my cats eat the starlings and sparrows [and rabbits] I'm afraid.
Though if I catch them after a native bird, I'll distract them. If I'm too
late and the bird in in their clutches, I take it off them. So far only one
lorikeet has been killed. But many starlings have been dispatched.