Pretty sure it’d be like taking their mind/magic away. He could probably become more powerful with it by stacking similar magics on top of one another (like absorbing a fire wizard’s magic and making it stronger by absorbing another fire wizard’s magic).
I don’t think he’s actually taking their mind but the magic from it. The mind is soil, a plant is magic, he doesn’t cut it from the stem but rips it out with the roots leaving the mind empty. Just my take though idk not a lore master.