All your magic must manifest through a specific mechanism. This can be anything, but must be fairly specific. This prevents you from creating certain effects; you just cannot work magic outside of what the limits on your manifestation allows. It forces you to be creative in the extreme to use certain effects, and your GM may not let you get away with certain things, no matter how creative you are.
Manifestation can also serve as a clue to who worked the magic - someone who knows your magic always manifests as fire won't have a hard time identifying your work.
Examples:
- A fire sorceress who must manifest all her spells through fire; healing is the burning away of damage flesh, divination is done by staring into fire or at flying sparks. It is impossible for her to create effects relating to water.
- A druid who must manifest all his magic as aspects of nature. Blasts are flying thorns, illusions are mirages formed by heat and mist. All his magic is hindered if there is no natural phenomena nearby to cause his effect.
- A technomage who must manifest her magic as the work of devices and technology; scrying is satellite downloads or surveillance cameras, blasts require gun props and so on.