Hi there, Andrew,
I think it has two functions, first to (help) hold the work-piece flat to the table and, second, to do that in a way that facilitates forward movement but opposes kick-back.
The modern equivalent, I think, is a plastic gizmo with parallel flat leaves, all slanting in the appropriate direction. The term 'feather-board' comes to mind. They are also recommended for use on router tables.
A browse on the Axminster site might show you some pix of the thing I've tried to describe.
I'd try to find you a pic and add it to this post but I'm eager to go and machine a back-plate for my recently acquired (pre-enjoyed) 4" Grip-Tru lathe chuck. Please excuse me.