They describe it as a “double loan and then a redemption.” That is to say, two seasons on loan with a loan fee, followed by a mandatory purchase by the Italians at the end of those two years. That allows the cost to be spread over several years, plus means Chelsea will take the burden of some of his wages for those first two seasons.
On Chelsea’s part, it allows them to obfuscate a loss-making operation in the confusion of the complex setup.