One example is if West leads a Q (top of a sequence) against a NT contract, and East plays K from Kx to unblock. If allowed to hold the trick, East would lead back the x.
My understanding is that bridge play is designed to protect certain vulnerable holdings, such as tenaces, in the declarer's hand from direct attack by the opening leader, West.
But if the tenace suit is led by East, its vulnerability will be fully exposed. Should East be on the lookout for potential vulnerable holdings in South, and overtake West's lead if an examination of his cards and dummy (and a review of the bidding) leads to a suspicion that South may have such a vulnerable holding? Here is an example of what I mean.