You’ve trained a skill to use an attribute other than its usual controlling score.
The GM decides what to allow, and the change should make sense.
Some guidelines:

　●DX-based skills with a complex technological aspect can shift to IQ (“Battlesuit based on IQ”), while those that involve fine work or hand-eye coordination can move to Per (“Driving based on Per”).

　●IQ-based skills for small tools can go over to DX (“Lockpicking based on DX”), those used to find or follow things can move to Per (“Shadowing based on Per”), and those with a supernatural angle can shift to Will (“Ritual Magic based on Will”).

　●HT-based skills for movement can shift to DX (“Swimming based on DX”), those with a social side can move to IQ (“Sex Appeal based on IQ”), and those for cinematic combat effects or resistance can become Will-based (“Body Control based on Will”).

　●Will-based skills can generally drift to IQ (“Exorcism based in IQ”), and those with cinematic combat effects or that aid resistance can shift to HT (“Power Blow based on HT”).

　●Per-based skills involving hand-eye coordination or fine manipulation can move to DX (“Search based on DX”), and most Per-based skills can shift to IQ (“Detect Lies based on IQ”).

　This affects routine skill use but doesn’t override a GM- or scenario-mandated roll based on a specific attribute (see p. B172). For instance, if an adventure calls for a Per-based Lockpicking roll, Attribute Substitution (Lockpicking based on DX) doesn’t let you make a DX-based roll ―― you have to make a Perbased roll like anybody else.

　You must specialize by skill and attribute, as in the examples.
Nobody may have more than four Attribute Substitution perks.

　Some wizards practice arts that enable them to learn variant spells based on attributes other than IQ.

　DX-based spells fit martial wizards whose art relies on whirling dances and complex hand or staff motions.
This option goes well with Magical Weapon Bond (p. 27) and No Incantations (p. 28).

　HT-based spells suit wizards whose shouted incantations demand excellent breath control, or who direct “inner mana.”
This often accompanies Afflicted Casting (above), Elixir Resistance (p. 24), No Gestures (p. 28), and similar “internal” Magic Perks.

　Will-based spells are ideal for casters who bend mana like psis bend spoons.
Mages like this may want Shaman’s Trance (p. 30) and Willful Casting (p. 33), too – and note that the latter remains useful for spells not bought with Attribute Substitution.

　Per-based spells work well for seers and those who can perceive the ebb and flow of mana – and for Information and Knowledge spells in general.
Magical Analysis (p. 26) frequently accompanies such ability.

　The GM may allow other variations.
He might even wish to explore Calculated Bases (Thaumatology, p. 30) for particularly bizarre styles of magic.

　You must specialize by spell and by attribute – it’s neither possible nor balanced for a single perk to affect all spells.
Appropriate examples would be Attribute Substitution (Aura based on Per), Attribute Substitution (Breathe Water based on HT), Attribute Substitution (Command based on Will), and Attribute Substitution (Deathtouch based on DX).
The GM decides how many Attribute Substitution perks each wizard may have.

　This perk changes the spell’s controlling attribute for all purposes, but not the scores used for ancillary rolls.
For instance, Attribute Substitution (Lightning based on HT) – quite suitable for mages who invoke Air magic through breath control – gives Lightning a HT-based roll to cast, but the roll to avoid disaster when injured is still against Will, and the attack roll is still against DX-based Innate Attack.
Other perks might modify these rolls, of course.