This is probably the best I've seen it described, without getting overly technical.
Until the car is over 50km/h it cannot deploy any of its electrical boost, meaning the initial getaway is reliant entirely on the V6 turbo engine.
Under the previous set of regulations an element of the power unit's hybrid system, known as the MGU-H, was used to spool the turbo to provide the optimum boost pressure for the start, but the MGU-H was removed as part of the changes to the power unit this year.
As a result, the V6 engine now needs to be revved to build up turbo boost pressure and minimize a common phenomenon in turbocharged engines known as turbo-lag.