On the 9th page of the next chapter it becomes explained as casting from HP instead of MP (a relatively common game mechanics, especially in roguelike games).
However, I wholeheartedly agree that such things look much better when explained (or at least foreshadowed) beforehand.
Hey I'm not tryna call you put or anything, but what games?
I'm admittedly old, but I've been playing video games since Super Mario Bros, and I'm ot sure I've ever played a video game with that mechanic.
I've allowed it for my players as a DM (and asked for it as a player) for D&D, WhiteWolf, ICE/LOTR, and Palladium; but... I've never played a game where it was standard.
Was this in PC games that I simply haven't played?
I'm curious because, while it's used in some manga, I've never played a game where that was actually a thing.