DRS is not trying to re-umpire every ball from zero. It checks the on-field decision and only overturns it when the evidence is strong enough under the review system.
A player review challenges an on-field decision and must be requested by an eligible player within the competition's time limit. The team has a limited review allocation, so the outcome can affect what remains later in the innings. An umpire review is initiated by the officials for situations their protocol permits, such as some run-out, stumping, boundary, or fair-catch questions. It does not depend on a captain or batter spending a player review. Broadcast graphics may label both sequences as a “review,” but identifying who initiated it explains the questions the third umpire may answer and whether a team's allocation is at risk.
For LBW, the third umpire first checks that the delivery was fair and whether the ball touched the bat before the pad. Ball-tracking then reports pitching, impact, and projected wicket contact in the order required by the protocol. A single failing element ends the dismissal. Umpire's call applies only to specified marginal ball-tracking zones and preserves the on-field decision for that element; it is not a general verdict that the whole appeal was half right. Under ICC protocols, a review returning umpire's call does not normally cost the team its review, even though the original decision remains unchanged.
Different questions call for different evidence: replays can locate a crease or boundary, edge-detection tools can help with bat contact, and tracking estimates the post-impact path. The third umpire works only with technology approved and available for that match. If a feed is inconclusive or a component fails, the protocol explains how to return to other evidence and, where the required proof is absent, to the on-field decision. DRS is therefore a structured correction system with a burden for overturning, not a promise that every close event will produce a new definitive answer.
An overturned decision can change more than the dismissal symbol. The protocol tells officials how to restore runs, place the batters, identify the next striker, and account for the delivery. Some actions after an erroneous out call may be treated as dead because play stopped at that call. The third umpire communicates the final decision to the on-field umpire, who signals it and manages the restart; the broadcast animation itself is not the official ruling.
Sometimes technology shows it was very close to out, but not clearly enough to erase the on-field call. That is exactly where umpire's call comes in.