Yup, more juggling.
Though, when you're just shifting numbers around, it's relatively easy to allow negative numbers.
Taking money from an account with zero and giving it to another account, thereby creating new electronic money from an account with nothing (Now is negative). Or rather taking from an account more than is there so it is negative.
Well, while this would be illegal (Unless it is allowed by law to a great surprise) I wouldn't be surprised if banks did do this since it's just electronic money. Clever bankers could find loopholes in the system to generate new money without anyone finding out. New money which could go into bank profits.
Or would they risk counterfeiting money? If they got found out, wouldn't it be really bad? Especially in the eyes of the public.
But the system od electronic fiat money is so screwed up, that I'm sure at least someone has abused or is at least trying or tried to abuse the system.