It also has a whole lot to do with how they cut the meat, as well as the flash cooking of a genuinely hot wok. (You may of course already know these things.)
I got a Rambo "wok burner" as part of the deal of us moving to a house with an electric stove (who was I to know it would work so well for brewing :blink: ). It made a massive difference to the tenderness of the meat in my stirfries. The meat is literally in the wok for no more than a minute, often 45 seconds. You want to make sure the meat doesn't end up cooking in its own juices or in the sauce. That toughens the fibers quicksmart, unless you're doing a long-cooked stew of course. Stirfry the meat first, take it out, then do the veggies, put the sauce in to heat and then stir the meat through and get it off the burner and onto the table ASAP.
You also need to cut the meat perpendicular to (across) the grain and slice it thin, so the actual length of the muscle fiber you're biting into is no more than the width of the slice. Allowing the meat to slightly freeze can help, but I never do this and have little trouble getting it thin enough. If your cut of meat has grain going a number of different ways then break it down into chunks that only have the grain going in one direction. THEN slice it across the grain for the stir fry.
Also, fully freezing meat can help break down fibers, and freezing it twice even more. Again, I don't bother to do this and am pretty happy with how tender my meat is
. If you do the double freeze be sure to thaw it in the fridge before re-freezing, not on the counter or in water.
I guarantee you that the vast majority of Asian restaurants are not using premium grades or cuts of meat. It has a whole lot to do with the cooking and the cutting.
Finally, another tenderizing method is to marinate the meat in milk for 24 hours. An Argentinian friend of mine turned me onto this. Seems to work pretty well but i reserve it for barbequing. Kiwi is awesome, but as I say above, not really necessary for meat in stir fries.