Roll-over is the weak ground ball to the pull side — the one where your top hand turns over too early and the barrel dies before it ever drives through the ball. You feel it as a swing that "gets around" the ball instead of through it, and the result is a steady diet of soft grounders to the pull-side infielder. It's fixable, and the fix is one of the most satisfying feels in hitting once it clicks.
Why the barrel exits early
Your two hands have two jobs through contact. The bottom hand pulls the knob and helps the barrel drive through the ball toward the pitcher; the top hand supports and then rounds after contact. Roll-over is the top hand taking over too soon — it turns the barrel face-down while the swing is still supposed to be extending through the ball, so the barrel cuts off short and the ball squirts out weakly to the pull side.
The instinct is to blame your hands or your bat. It's neither. It's timing between the hands, and it's coachable.
What the camera reads
Film open side and CoachVision measures your extension through the ball — how far your lead arm drives through contact versus your own fully straight arm. A roll-over cuts that short, so extension reads low: the arm folded and the barrel exited before it drove through. It's self-calibrated to your own arm length, so there's no absolute number to argue about — just "long through the zone" versus "cut off early."
Here's the honest part you should hear plainly: the camera can not see your bat, your grip, your top-hand release, or the ball. So it doesn't measure "you rolled over" directly — it measures that your arm didn't extend through the ball, which is roll-over's fingerprint. The frames fill in the picture; the number reads the drive. And "contact" is your hands' speed peak, not bat-on-ball — this is a swing-shape read, not a verdict on where the grounder went.
The fix — teach the hands their jobs
Top-Hand / Bottom-Hand Tee Work is built for exactly this. Take one-handed swings with a short bat or a choked-up grip. The bottom hand learns to pull the knob and drive through; the top hand, isolated, learns to support and round late instead of taking over early. When you put them back together, the top hand has learned to wait. Eight swings each hand.
The cue when you go back to two hands: stay through the ball with the palm up a beat longer — feel like the top-hand palm faces the sky through contact and only rolls after you've driven through. "Hit the inside-back of the ball and keep driving toward the pitcher."
Film one open-side swing, check your extension, then eight top-hand/bottom-hand swings a round until the barrel stops turning over early and starts driving through.