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Is there a rule for lowering the Mystic def. for friendly spells?

Not as written. This is because of the new spell systems using extra successes.

A couple of house rules I've heard of is the ability to lower Mystic Defense as allowed in previous editions, but extra successes are always calculated off the character's 'normal' Mystic Defense, or that extra successes do not apply if the target willingly submits to the spell.

(I prefer the first option, as it retains most of the utility without allowing a lowered defense to grant free successes on the test.)

You can use the combat option "Aggressive Attack" and get a -3 penalty to Mystic Def.

Would that combine with a knockdown penalty? You could take an aggressive pratfall.

"Tales abound of the t'skrang Swordmaster/Troubadour B'ster K'ton, known for dazzling the audience -- and dizzying his foes -- with acrobatic feats that were intended to make it look like he was in mortal danger, but would somehow end up with him coming out golden."