Marc Matsumoto has a great post on tenderizing meats:

Whether it’s a tender juicy steak or a fall-off-the-bone tender rib, tenderness is often equated with the quality of the meat. But the texture of meat has a lot less to do with quality, and a lot more to do with how the meat is treated both before and after hitting the grill. To better understand this, let’s look at the three main methods of tenderizing meat: mechanical, thermal, and enzymatic.

Mechanical tenderization involves pounding or piercing the meat with one of those medieval looking devices. The physical action is essentially pre-chewing the meat for you. I’m generally not a big fan of this technique because you’re not only breaking up the connective tissues that make meat tough; you’re also breaking up the meat fibers themselves.