Soft Starting
This feature is a standard VFD feature and when properly set-up, it will not only extend the life of the motor but also reduces its power usage and the deadly effect on the whole distribution system across the line.
Speed Control on the Input
It’s really application-specific. You still want the gearbox for torque multiplication, and something a variable frequency drive can’t give you. You can save power if you simply reduce the motor speed rather than dissipating the work already done by the motor.
Continuous Drive Instead of ON/OFF
This is also an application-specific; for example, a motor is oversized to meet some idea of a maximum load but in reality, is never there. The motor cycles on and off and the variable frequency drive effectively right-size the motor to the load, actually driving by throttling it back.
A VFD is able to save power with the help of the above effective ways and it saves power because it consumes the power that’s needed across different application usage points.