This model can be used to vary the duty cycle of PWM by varying a constant value. Here, duty cycle depends on a constant value which is related to the duty cycle by the given relation. If d is duty cycle and the constant value which will vary the duty cycle is a then, d=(4-a)/8. For example, if you want to create a PWM with duty cycle of 25% or d=0.25 then a will be 2. 'a' may have any value between -4 to 4 depending on the required duty cycle. As the value of a increases, value of d decreases.
Phase shifting is also illustrated in the model.