The Waveform Generator uses the TCCR1A.COM1x[1:0] bits differently in Normal, CTC, and PWM modes. For all modes, setting the TCCR1A.COM1x[1:0]=0x0 tells the Waveform Generator that no action on the OC1x Register is to be performed on the next compare match. Refer also to the descriptions of the output modes.
A change of the TCCR1A.COM1x[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 TCCR1C.FOC1x strobe bits.