When a sine wave is mixed with one of identical frequency but opposite amplitude (ie: of an inverse polarity), the combined result is silence. A two-channel stereo recording contains a number of waveforms; sounds that are panned to the extreme left or right will contain the greatest difference in amplitude between the two channels, while those towards the centre will contain the smallest. A mix of the left channel with the polar inverse of the right channel will reduce centre-panned sounds towards silence, while preserving those towards the extremities.
In practice, the OOPS technique can be performed by inverting the polarity of one speaker or signal lead. It can also be performed using digital audio software such as Audacity by inverting one of the channels of a stereo audio waveform, and then summing both channels together to create a single mono channel.