How to derive this formula from this schematic?
Study this picture of a diode clamp: -



As you can see an AC square wave is converted to a mainly positive square wave by use of a diode. The resistor is very high in value and prevents the output rising positively too high but can be ignored.

Given that you have a stage of rectification using a similar "diode" there are two diode drops to consider so, instead of 2V-Vo as per the picture you will have 2(V-Vo).

Returning to the basic clamper circuit, the diode could in fact be connected to a DC voltage source of Vx volts and then you should be able to see how the formula in your question relates.

Instead of diodes MOSFETs are used but they achieve the same thing.