"...So you're saying it could destroy us both?"
"Right, it would be out of the question for me to take your Magic Circuits, and giving my Magic Circuits to you would be a big mistake.
So---"
"So?"
"I'll implant my Magic Crest into you."
---A Magic Crest.
A bloodline's history, in which all its secrets and mysteries are written.
If Magic Circuits are blood and bone, then the Magic Crest would be the flesh.
A Magic Crest is magic given form.
The proof of a single magus's existence, as the family's head spends a lifetime giving intangible magic a form
As the inheritor of a Magic Crest, Tohsaka can use the magic given shape by the past family heads
"...But that's---"
"If I'm going to give something away, it's much easier if it's the Magic Crest. After all, unlike a Magic Circuit, it exists to be passed on.
...Well, it's not really a part of me the way my Magic Circuits are, but it's made from the Tohsaka lineage. It'll do just fine for a path."
"...Are you sure?"
"Sure I'm sure. The magic that makes up the crest, well, it'll become useless, but I'll be able to convert it into a receiver.
Oh, or are you asking about the success rate? If I'll be able to transplant the Magic Crest to someone outside of the family?"