A [Spellcaster] may attempt to weave a [Thread] to a [Spell] if the [Spell] is cast with [Raw Magic], or if the [Spell] is in a [Spell Matrix] and the size of the [Spell] plus the size of the [Thread] being weaved is no bigger then the [Rank] and thus the size of the [Spell Matrix].

[Known Threads for Spellcasting] 

Thread Weaving
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If you have spare space in a matrix, you can use it for extra threads.
These can do things like increase range, make a spell area effect, and
so on.  Each open point of matrix size can hold one thread.  Threads
may be woven in advance.  They're cleared if the matrix collapses, but
otherwise stick around.

If you're good enough to design your own spells, you can take an
existing spell and design a new version with threads pre-woven in.  So
instead of a size 1 "Fireball" which does 1d + 1d per thread woven in,
you could design a size 6 fireball which does 6d +6d per thread woven
in... but it would be a Circle 6 spell unless you're *really* good.

ThreadWeaving is used in the ThreadWeavingMagic mechanic.