I took a fantastic class with Cynthia Guffey on piping. It was one of those 2.5 hour evening classes. There were problems with the sewing machines (wrong feet and clogged with thread from previous class) and so Cynthia sat down on the floor and walked us through the process by hand. And, it was even better than it would have been if the machines had been working great. We could watch her hands close-up and see just how she achieves perfect results. I will never reach her level of skill, but my knowledge grows each time I take one of her classes.
Here is one tip she offered: never understitch by machine on a concave curve. Even if you have trimmed and clipped according to Cynthia's techniques, the machine will close the clips and cause a bubble with the seam is in place. She recommends hand-stitching the understitching to maintain the curve gained through clipping.