In a parallel series circuit if the resistors that are split across the parallel section of the circuit are equal (R1=R2) you may divide the value of one of the resistors by the number of the resistors (R1/2) to find a resistor that can replace the parallel section of the circuit (R3) while keeping all the values of the circuit outside of the parallel section of the circuit the same.
This circuit models this principle by allowing you to switch between the parallel section of the circuit (R1 and R2) to the resistor that has been calculated to maintain the values (R3)
You'll notice that this is done twice with two different resistor values. As long as the resistors you are trying to combine are parallel and have the same value you can simply divide the value of those resistors with the number of resistors you are splitting to find the parallel equivalent resistor value.
