It’s a system, introduced to Test cricket in November 2009, that allows teams to challenge the on-field umpire’s decision using the same technology that has been available to television broadcasters and viewers for years.
Under the system players can request an on-field umpire’s decision be reviewed by a third umpire with all the available technology at his fingertips. The system operates in a similar manner to tennis’ hawk-eye line-call challenges, with each team being allowed a maximum of two unsuccessful requests for review per innings.
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