In your .bp File your part has an x,y and z orientation set. Now, when you flip, you turn one positive x or y value to negative, or negative to positive, depending on initial rotation value, flip direction and positive or negative initial state of the relevant x or y value. z is your rotation value.
That means that since you start with a zero state on all three values, you can predict and manipulate them without .bp editing. A 0/0/0 value set will look the same as a -1/-1/180 on screen. But they attach quite differently in the code. Going to launch and back to build resets all values back to zero for newly placed parts.