Robin, I suspect he is talking about something like
http://www.starlight-xpress.co.uk/OAG.htm
Frank, I have one of those and am just in the process of getting it set up. They use a plane-parallel chunk of glass, 12mm thick that the 'twist' with 4 servo motors under control of a guide camera. The light passing through it to the camera is thus slightly deviated. A much quicker response to guiding commands than trying to move the whole mount as I normally do.
Charles