how are spell effects harnessed to create more spells ???
If I remember correctly, Mysticism is a school that cannot be fully understood, and its rules change constantly. It's a completely incomprehensible list of paradoxes and contradictions.
In fact, Mysticism is something that may even be powered by not being able to understand how it works, or even the spells that didn't work.
I'd think of it like trying to explain to somebody how what the sense of "taste" is like, if they've been born without tastebuds. You can't really describe what taste is, but you know it at the same time. You can't really explain how the school of Mysticism works, assuming you're talking about spell absorption here.