Substitute creates a small copy of your Pokémon. Creating a substitute
removes 25% of your Pokémon's original HP; the sub can suffer exactly that
many points of damage before it breaks. Once it is created, and until it
breaks, all damaging attacks on your Pokémon hit the sub instead. On the
turn that a sub breaks, no damage spills over to your Pokémon (eg, if the
sub only has 20 HP, and it is hit by a Hyper
Beam doing 200 points of damage, your Pokémon will still take no
damage). (See the entry on Hyper Beam for more information on its
interaction with substitutes in the RBY game.)
A sub has nearly all of the same properties as the Pokémon that created
it. Its statistics (other than HP) are the same, although this is not
relevant for Attack, Speed, or the offensive aspect of Special, since subs
do not actually attack. (Your Pokémon can still attack, however: the sub
simply moves out of the way momentarily.)
Any modifications to your Pokémon's statistics at the time the sub is
created are also present in the sub. Once the sub is created,
statistic-modifying attacks (such as Screech and the Speed-lowering effect of Bubblebeam) by enemy Pokémon cannot affect
your Pokémon or the sub, but self-modifiers (such as Amnesia) will apply to both your Pokémon and
the sub.
Subs have the same type(s) as the Pokémon that create them.
Subs cannot have their status changed. If a poisoned, burnt, or paralyzed Pokémon creates a sub, the sub
itself will not suffer from any of those effects. (Note that this is a
moot point in the case of paralyzation: your Pokémon still suffers the
Speed reduction and will occasionally be fully paralyzed.)
Subs are also immune to status-changes after creation. However, the
protection they provide to their creators is limited: Pokémon with subs
can still be put to sleep and paralyzed.
Disable and Leech Seed also ignore substitutes (Leech
Seed draws HP from the Pokémon, not the sub). Recoil damage from moves
like Submission is also taken by the
Pokémon. Haze affects the Pokémon, not the
sub.
On the other hand, subs protect their creators from poison, burns, and
freezing. However, if a burnt or poisoned Pokémon creates a sub, the
Pokémon continues to take damage: none of it is absorbed by the sub. Confusion attacks (Confuse Ray, Supersonic) also fail against subs;
however, Pokémon with subs can still confuse themselves using Petal Dance or Thrash. All self-inflicted confusion damage is
taken by the Pokémon, not the sub. A successful One-hit KO attack on a sub simply breaks it; the
Pokémon suffers no damage.
The damage absorbed by the sub can be released with Bide.
If Explosion or Selfdestruct is used against a sub, and
causes it to break, the Exploding/Destructing Pokémon does not faint, as
would normally be the case. Its picture will no longer be visible, but it
can continue to fight normally.