If you lay out a flank, you can clearly see a "grain" running through the meat. Bundles of muscle fibers are closely packed and run parallel through the meat. They look like little red tubes. These muscle fibers did a lot of work for the animal during its life so they are tough as can be.
After cooking theses cuts to no more than medium, with medium rare preferred, let the meat rest so the cells in the muscle can re-absorb the juices squeezed out during cooking. Then, slice the steak perpendicular to the long muscle fiber bundles. Perpendicular slicing cuts off small pieces of these 'tubes' or bundles.
Cutting across the grain like this dramatically weakens the integrity if the bundle, making it a lot easier for your teeth to finish the job.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan