The deal is apparently that apart from amusing you and all your neighbours, and putting you in severe danger of your cat never talking to you again, it stops cats from catching birds. Partly because it's large and bright and warns them off, partly because it gets in the way of their final pounce.

I wouldn't have believed it if somebody told me, but there are in fact pictures of 200 (200!!) cats blithely wearing the thing on there.