The AutoPanner is a very well-known effect; its hardware incarnation originates in the age of voltage controlled synthesizers. Its main use is to liven up synth tracks in the mix.
The "Depth" control sets the wideness of panning. A setting of 100% means that the mono input signal is panned between the extreme left and right positions. If you apply the plugin to a stereo track, it behaves as if you were adjusting the balance control on your home stereo (the left and right channels are not mixed together, but their gains are controlled separately). If you feel that the overall loudness of the track decreases when this effect is applied, you can compensate this by setting the "Gain" control.
This plugin was implemented as an enhancement of the TAP Tremolo plugin: processing was extended to 2 tracks, with the LFO-s phase-shifted to be in counter-phase.