How does Account Leasing work in the Peercoin rival Nxtcoin?

Link: http://nxter.org/its-forging-time-nxters/
Account leasing in Nxtcoin is very confusing. It’s like loaning money to somebody while still keeping it in the purse. Bizarre. That is equivalent to creating money by fractional reserve system in the Banking industry.
Anyone can explain it mathematically, logically or from the computer science standpoint?