The waveform generator uses the TCCRnA.COMnx[1:0] bits differently in normal, CTC, and PWM modes. For all modes, setting the TCCRnA.COMnx[1:0] = 0 tells the waveform generator that no action on the OCnx register is to be performed on the next compare match. Refer also to the descriptions of the output modes.
A change of the TCCRnA.COMnx[1:0] bits state will have effect at the first compare match after the bits are written. For non-PWM modes, the action can be forced to have immediate effect by using the TCCRnC.FOCnx strobe bits.