A leg before wicket review looks like a machine overruling an umpire, but the process is narrower than that. Part of it is measurement and part of it is projection.
Three questions have to be answered in order
The review establishes where the ball pitched, where it struck the batter, and where it would have gone afterwards. Each question can end the review on its own.
If the ball pitched outside leg stump, the appeal fails immediately regardless of everything else. If the impact was outside the line of off stump while a shot was offered, it fails too.
Only when those two conditions are satisfied does the projected path matter, which is why many reviews are resolved without the predictive element being decisive.
Measurement and prediction are different kinds of evidence
Pitching point and impact point are observed. Cameras record where the ball actually was, and the system reports positions rather than estimates.
The path beyond the pad is a projection. The ball never travelled it, so the system extrapolates from the trajectory it observed before impact.
Because a projection carries uncertainty that grows with distance, a ball struck well in front of the stumps is predicted less confidently than one struck close to them.
Umpire's call exists to respect that uncertainty
When the projection shows only a marginal clipping of the stumps, the original decision stands. This is not indecision; it is the system declining to overturn within its own error margin.
The consequence is that identical ball-tracking output produces opposite results depending on what the umpire said, which is the part spectators find hardest to accept.
It also gives the on-field decision real weight, and it is why bowling and batting sides treat the umpire's initial signal as information when deciding whether to review at all.
Reviews are a limited resource
Each side holds a small number of unsuccessful reviews per innings. Spending them badly leaves a team unable to challenge a genuine error later.
This turns reviewing into a judgement under time pressure, made by players who saw the delivery from poor angles and have seconds to consult.
Batters at the striker's end have the best information about impact but the worst about height, while the non-striker sees the line clearly and the contact not at all.
The technology changed how bowlers attack
Before review, deliveries striking the pad in front of the stumps from around the wicket were rarely given, and bowlers adjusted their lines accordingly.
Once projections could demonstrate that such deliveries were hitting, umpires began raising the finger more often, and spin bowlers in particular gained a dismissal that had been largely unavailable.
Batting technique followed. Using the pad as a defensive tool became far riskier, and playing with bat close to pad became the safer default.
