If the the plug isn't fully inserted, the switch may or may not be moved, and the matching metal contacts might not be aligned properly.

In addition the metal contacts that make up the shaft of the plug end, in this instance ground, left channel, and right channel separated by plastic insulators, also have to line up with their corresponding ground, left channel, and right channel contacts inside the jack. The plug end pushes aside a spring-loaded piece of metal, switching contacts so the audio out changes from the speaker to the headphone/earbud. To be honest I really can't say, but when you push your headphone plug end into your phone's headphone jack, it really is just a mechanical switch inside.