It's called voluntary stuttering and like many have said, it's purpose is to get the user to stuttering on feared words or sounds in an effort to overcome them. I do need to point something out though. This isn't a miracle technique that you can just start using to try and keep your stuttering under control. Jumping right into voluntary stuttering can yield positive results at the start, but it's likely to fade just like other "tricks" or techniques that work great when you first start them and then fade. Voluntary stuttering isn't a control technique or an antithetical fluency technique. Using as such will likely make it a new trick youre adding to your speaking behavior with a novelty effect that will come down in time.