[quote=GuyYouWishYouDidntKnow]
After that it's a bit of [logic], but that's about all I've got quote-wise. Knowing BB, he probably has something more substantive.
[/quote]

[url=http://lparchive.org/Fatestay-night/Update%20266/]
[quote]Taking the false Servant's flesh as its own, the true Assassin enters our world.
"KiKiki, kikikikiki"
His first cries sound like those of an insect.
The black insect that crawled from the swordsman's stomach devours the body that was his own nursery.

He giggles while tearing the flesh apart.
He laughs while crunching on the bones.

Every time he does so, the insect becomes more like a man, and the empty brain acquires human wisdom.
And after a while…
He has finished sipping every last bit of the blood and blesses the birth of his own self, the "assassin".[/quote]

I'm fairly certain (not 100% sure) it's only relevant to his intelligence, based on him eating Assassin too.