Magic is a means of forcing chi energy to obey your will, and chi energy does not like to be forced. Sometimes a sorcerer ends up channeling more energy than he can handle. This makes magic inherently dangerous in a way that using fu powers or firing guns is not. When a character fumbles any Sorcery check, he suffers a magical Backlash. Backlash effects vary from schtick to schtick, and are explained in the schtick descriptions later in this chapter.
If your fumble was the result of a boxcars failure, backlash is the only side effect. If it happened because your Action Result was below zero, your spell also goes awry, with the usual GM-determined fumble effects.
Yuan Guo is casting a Blast spell at a rival sorcerer. He makes a Sorcery check with a Difficulty of 13. His Action Value is 15. He gets boxcars on his first roll. He then rolls a -3, for an Action Result of 12-a failure. This failure following double sixes means that he suffers a Backlash.