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Deals damage and has a 10% chance to flinch the target.
Extrasensory is hardly seen competitively. Pokemon with access to this move either don't need it for coverage, as is the case for Zoroark, or have stronger Psychic-type moves to use, as is the case for many Psychic-types. The 10% chance of a flinch is too small to warrant the use of Extrasensory over, say, Psychic, which is 10 Base Power stronger, or Psyshock, which hits the opponent's Defense stat.
Psychic is a stronger alternative that has a chance to lower the opponent's Special Defense, but it doesn't have a chance to flinch.
Psyshock is an equivalent, except that it hits the opponent's Defense stat and does not have a chance to flinch.