Skirt or Flank steak have long meat fibers. When you cut with the grain you will be chewing those long fibers for a long time. So, take a very sharp knife and cut thin slices across the grain. These shorter fibers will chew easily and fall apart in your mouth - something we refer to as tenderness. While cutting on the bias or with the grain might be more appealing on the plate it won't make for good eating. When it comes to sirloin the grain runs up and down rather than across as on a skirt or flank steak. So with sirloin cut on the bias to keep those meat fibers as short as possible.