How to do it
Look carefully at the top of the meat. You should be able to see some of the grain (almost like very tiny stripes) running across it. Using a very sharp knife, slice the meat perpindicular, or at a 90 degree angle to the grain. So it would be like slicing the stripes in half, rather than slicing with the stripes.
Why it works
The thin strings of individual muscle sinews are held together by a tough material and by cutting through it, rather than cutting with it, you break it down before it goes into your mouth.