Every time the caster does a melee attack (which does NOT have to hit) there is a chance that he automatically casts a certain spell. The Sage must know the autocasted spell, and the SP Cost for them is 2/3 the normal cost.
If a chosen skill is level 3, Auto Spell casts the skill with a level of 1 to 3 randomly with a chance of casting the level 1 version 50% of the time, the level 2 version 35% of the time, and the level 3 version 15% of the time.

Autocasts certain magic spells by a certain chance while attacking enemies with physical attack.
Spells that are auto cast through Auto Spell only consume 2/3 of the required SP and are cast at half of the skill level of Auto Spell.
The amount of spells which will be casted and the chance to autocast depends on your Auto Spells skill level.

If the learned skill level of a spell is lower than the half of Auto Spell, autocast the learned skill level instead.
Spells that are not learned by the caster can't be casted.