The rule governs the ability of the cross-examiner to introduce extrinsic evidence in his or her case to contradict answers given by an opposing witness on a collateral issue: R v Khanna, 2016 ONCA 39 at para 9; R v MacIsaac, 2017 ONCA 172 at para 58
If the questioner asks a question that bears on a collateral issue, he or she is “stuck” with the answer, in the sense of not being permitted to lead extrinsic evidence to contradict it. However, this does not prevent proper questions from being put in the first place: MacIsaac at para 58.
Generally speaking, credibility is considered to be collateral, thereby barring the questioner from adducing extrinsic evidence that bears solely on this issue. However, the rule has developed in a manner that admits of a number of exceptions. For example, medical evidence may be adduced to prove that, by virtue of a mental or physical condition, the witness is incapable of telling or is unlikely to tell the truth: MacIsaac at paras 59-60; see R v SB, 2016 NLCA 20 at paras 11-21, aff'd on appeal at 2017 ONCA 16
The rule does not regulate cross-examination of an opposing witness about prior utterances that contradict his or her testimony on a collateral matter: Khanna at para 9