I just tried one of these with Ellie and it was a horrible experience. She barks out of boredom but also over the vacuum and things like that from anxiety. You fill the little canister with citronella and attach it to the collar and put it on your dog. It sprays a gentle mist at their face when they bark and makes the spray sound. The smell is supposed to stop them from barking but Ellie seemed to want to sniff the citronella.
I got the broom out (an anxiety stressor) and she barked and it sprayed and she barked and it sprayed......... I think she thought the broom was spraying her so she barked more. It was miserable and it didn't work for us at all. I believe it could work for boredom barking and I know some people have had success but not us.