Not sure. Maybe.
Maybe the way it's notated is confusing me. See if I get it, now...
If an 8th Level magic-user puts "Minimal Will" or Effort into a spell, there's no danger if they fail, they will get a Partial Casting if they roll an 8 or above, and a Full Casting on a 12 or higher?
If they decide to put "Total Will" or Effort into the spell, it's easier to cast and they're more likely to manifest a Full Casting...but a failure will result in horrible consequences?