Stefflus you mention yaw in away your right,but the spindle moulder does not move, but the stock does up and down.
your comment made me realise that I had not posted pics with the
stock fastened to the template riding on the dowels.So here you go.
Mr Woodworking thanks for the encouragement puzzling well yes and no.Like most clever things it`s really very simple,the key to it all is to not think in lines but in bands(the width of the knives).
I thought the best way to explain how I think it was all done was to make some drawings of the process.
Fist drawing shows the shape produced on the first pass with the stock fastened to the template.
The template is then removed and fastened to the other side of the stock, but not in the same position it`s moved along slightly this is impotent to understand.
So in affect what happens now is the stock is up side down and fed past the spindle moulder.
Really clever the second pass not only produces a profile but is used to remove part of the original profile,there by creating a completely different profile (IE the feather).
The next drawing shows the path of the top of the knife as it pass`s over the original profile.The shaded area is the part of the original profile that is removed.
Both knives not only produce a wave at the centre of the stock but also towards the outside edge of the stock.
This wave is impotent because it determines where the sunken bead and edge profile will be placed on the stock.
I shall have to make another post because I`v posted the max pics on this, one so back in a min.