phase shifting rsg using variacs

Hi all,

 Not long ago someone touched on using a variac to vary the timing on a
RSG by shifting the phase of the power feed to the RSG motor relative to
the original waveform from the wall plug. How exactly is this done as I
would like to include it in my control unit if possible.

Power in OZ : 240 Vac 50 Hz.
I am assuming sync RSG's here.

Kind Regards,

 Robin Copini in OZ.