Leg before wicket decisions rest on a prediction about a path the ball never travelled. Tracking systems estimate that path, and the review protocol is built around the uncertainty involved.
What the cameras actually capture
Several high-speed cameras positioned around the ground record the ball in flight from different angles. Combining those views fixes the ball's position in three dimensions at each frame.
From a sequence of positions the system derives velocity, direction and the curvature caused by swing or spin. Only the portion of flight before impact is observed directly.
Everything after the ball strikes the pad is extrapolation. The system continues the measured trajectory forward using the physics established during the observed phase.
Why the prediction carries error
Camera resolution, frame rate and the precision of ball detection all introduce small measurement errors. Those errors compound as the predicted path extends further from the point of impact.
A ball striking the pad close to the stumps needs a short prediction, so the uncertainty stays small. Impact well down the pitch requires a longer extrapolation and a wider margin of error.
Deviation off the surface adds further difficulty, because a ball that pitched only moments before impact provides little observed evidence of how much it was turning.
How the umpire's call margin works
Decision review protocols retain the on-field verdict when the predicted path only clips the stumps. This is not deference for its own sake but an acknowledgement of prediction error.
The margin means a marginal projection cannot overturn a human judgement in either direction. Clear evidence is required, which matches the standard the review system was designed to apply.
Because the same margin applies to both outcomes, an identical trajectory can produce opposite results depending on the original decision. That asymmetry is the most argued feature of the system.
Conditions the system must be calibrated for
Tracking accuracy depends on stable camera positions and known ground dimensions. Installations are calibrated before play and checked when equipment is moved.
Poor light, obstructed sightlines or an unusual delivery angle can reduce confidence in a particular reading. Operators can declare that tracking is unavailable for a specific delivery.
Not every venue carries the full system, which is why review protocols vary between competitions. Available technology, rather than the laws, sets what can be reviewed.
What the technology has changed on the field
Umpires now give more decisions out to deliveries pitching outside off stump and straightening, because tracking has demonstrated how often those hit the stumps.
Batters have adapted by using the pad less as a defensive tool. Playing with bat closer to pad removes the ambiguity the review system resolves.
The net effect is a shift in what counts as a plausible dismissal. Technology altered the shared understanding of the law rather than the wording of it.
