follow the picture on typemoon wikia ( Fate/Complete Material III). They call both Od and Mana is " Life Force"
https://static.wikia.nocookie.net/ty...20111018161141
And Magic Circuits help to convert them to Magical Energy (魔力, Maryoku).
If follow this way, everything in FSN will easier to understand.
Thinks about Od and Mana as Crude oil. And Magical Energy as Gasoline.
So, when they having sex, they actually transfer "magical energy", not " mana", not "Od". Because Nasu keeps confusing when use the words" Mana, Od is is life force/Magical Energy...
It exists very many method to convert Mana, Od to ME. Medea converts mana from the air. Waver and Iris draw the huge magic circle on the ground, to consume Mana from the soil.
Autrum use machine to consume victims' Od and convert it to "magical energy".
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But. Still the question:
So, Kuzuki actually doesn't transfer any Od/soul/magical energy to Medea ( when they have sex with each other)? Why Nasu doesn't even care to explain it clearly in FSN novel???