It's a silent prolongation. Other people have called this a 'block,' an accurate description -- you're having trouble initiating the motor movements for the upcoming sound. One of the features of stuttering is difficulty initiating speech. So I'm not surprised that this predominantly happens on the first sound. Another hallmark of stuttering is its variable nature: the severity is not static. For someone with very mild stuttering severity, the stuttering could disappear from time to time. I do recommend following u/electric_dolphin 's advice on not relying on this "running start" strategy. If it's a functional gimmick that works from time to time, sure--that's great we all have those we use. But there is a risk of becoming dependent on it, and even if it works, it's going to become distracting to your listener. As such there may come a time when you don't want to use that /oi/ running start. (Undoing something like that is harder than you think).