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Takes 1/4 of the user's max HP to create a Substitute that takes damage for the user.
A Substitute does not block status inducing moves other than poison, confusion, the effect of partial trapping moves, or secondary effect confusion. It also blocks the effects of stat reducing moves and Leech Seed. If Selfdestruct, Hyper Beam, or Explosion are used on a Substitute, and the Substitute breaks, the consequence of the attack does not happen. When an HP draining move breaks a Substitute, HP isn't drained.
Not as good as in later generations, the inability to block status make it a poor move overall in RBY. Its main user is Rhydon, who can force a switch on Zapdos and set up a Substitute to prevent being KOed by the incoming Blizzard, and can thus severely damage its counter before switching out.