As souffle says, we call it pancaking, or splatting!
Im sure it just means that he is not quite sure about what may be happening.
Even though some of our hams are quite an age and love their strokes and cuddles, they sometimes do this. I seem to think they do this when they are 'enduring' the stroking, rather than enjoying it. It usually seems that mine do this when they want to do something else, like climb and explore, and me stroking them is just stopping them doing it!!