Thank you!
That's just a way of saying that you can't cast spells with 1 die in the mana pool. It's mostly intended to make specializing in 1/2 mana pools have a bit of a downside (you can't use spells of other mana types that you have 1 in), but can also of course apply to a more depleted pool. In that way, I would recommend to a player to think about the result of their spell, in that, if they want to do fire magic, obviously a red thing, but has only 1 red die, they'll need to either reskin it or provide a bit of an argument as to how their fire magic's intent is something more aligned with a different mana pool.
Hope that helps! If I revisit Zauberspiel like I did Anime Messerspiel, I'm considering a way to burn that last 1 die in a pool, since I can see it to be a bit annoying for players occasionally, since the only way to get rid of the die is to roll it currently.