Probably the easiest way to describe it is as follows. 1. Make up a
sandwich; backer plate, material for pattern (let’s use hardware
cloth with 1/4" openings in the example), metal to be patterned, 4
layers of paper towel. 2. Make a dead pass through the mill. 3. Close
the mill enough to cause the metal to be embossed (on my mill about
1/2 turn). 4. Roll the sandwich through the mill.
When the sandwich is opened, the pattern will have been pressed more
deeply into the metal than if the ‘pusher’ were not used. I haven’t
seen any scientific proof of what happens, but here’s my guess.
The ‘pusher’ is soft enough to be displaced where the pattern
material is. However, in areas where the metal isn’t supported by the
pattern material, the metal being patterned is depressed a little
more, (somewhat less than the 4 thickness’ of ‘pusher’).
The effect of the ‘pusher’ is a more defined pattern. An interesting
experiment might be to make up a sandwich about 6" long. Leave the 1st
3" without the ‘pusher’, then add the ‘pusher’ to the last 3" before
putting the whole thing through the mill. If you do the experiment
give us a report.
Hope this helps.