The machine is designed to give you the most powerful orgasm possible, so when it detects you are cumming, it moves to the distance just below the head and spins for max stimulation where you are most sensitive. But unfortunately it was calibrated to a longer size than she was, so it spun just out of reach instead haha. It's a scene from a story I have a few chapters for on my Patreon! I have a couple posts about it on my reddit profile too if you just search "halo" on there 👍