The machines without dual feed (or whatever it's called) - the bottom layer next to the feed dogs will feed through just a bit faster than the top layer.
Which is why the layer to be eased would be on the bottom (such as a sleeve cap).
Or when sewing knits when one seam was longer than the other - the top layer would be stretched and the bottom layer would be 'relaxed' - we were told to 'sew with a relaxed bottom'