When the split turbo strategy first came to light, some "in the business" we discussed the subject with were not convinced it alone was enough to explain the performance gap. The big advantage for the Mercs, they argued, is in their energy store charging strategy which allows them to not have to coast at the end of the straights to recover energy, like Renaults for example ("please match he beeps" remember that radio message to Daniel Ricciardo?). If that's not enough, the Mercedes unit also has an advantage in drivability and power.
These two characteristics would seem to be incompatible: If Merc is not coasting, they must get a lot of their charge from the MGU-H, so a big turbo. But a big turbo means turbo lag and more energy spent trying to spin it when it is being driven. And it is here that it's thought Andy Cowell and the team in Brackley came up with another brilliant engineering detail.
According to ex Ferrari engineer Claudio Lombardi in an interview on SkySportsF1 Italy, Mercedes have in place a system which can momentarily de-couple the turbine side from the compressor side. This allows the compressor to be free of the resistance of the turbine when it is being driven by electric power, making it more responsive when delivering power on corner exit.
The advantages of a small turbo while using a big turbo to produce more power and energy.