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Guarantees that the next move used by the user will not miss the target, even if the target has used a move like Fly or Dig. Effect is lost if either the user or the target switch out before the move is used. Does not allow user to hit through Protect or Detect. Works with OHKO moves. Fails if the user tries to use this move again during effect.
Lock-On has little competitive use. Low accuracy moves are rarely used, and telegraphing their use with Lock-On gives the target ample time to switch out. There are much more useful choices for a free moveslot.
Mind Reader is an exact equivalent.