The burner pauses briefly during burning (i.e. the dips you see in the burn graph) to check if the burn quality is good and makes corrections if necessary. This is done by the drive/firmware and the software has no control over it.
The OPC option in ImgBurn's settings asks for the OPC information before burning, and if this errors out it could indicate incompatibilty between your burner/firmware/media combination, but in practice it proved to be not that reliable, so that's why it's disabled by default. Notice though that this option doesn't affect the burner's internal OPC.