My course

Hi there! This is a special case where phase shifts can be identical to a reflection.

Change the constant to $\frac{\pi}{2},\frac{\pi}{3},\frac{3\pi}{2}$ and see that we won't be able to produce a 'reflection' for any constants.

why does it make the function reflect

Hi there! Thank you for your concern. At 12:40, we have -(-1)(-21) = -21, likely you omitted one negative.