A backfire occurs when a magic-user CASTs a spell with too many mana streams. The energy can come either from the initial PREPARE, from HARNESSing afterwards, or tapping too much energy from cambrinth or other stores. The primary limit to the amount a caster is capable of using is the appropriate magic skill, such as Debilitation for curses, modified by their Primary Magic skill skill.
Backfiring will occur more if you are fatigued.
Normally backfiring simply results in the spell failing. However, in extreme cases, it will causes damage to the casters nervous system, which is felt as a tingling sensation.