According to AmazingRibs.com, written by Craig "Meathead" Goldwyn (and how do you not trust a guy named "Meathead"?), "it is the tensor fasciae latae
muscle from the bottom sirloin, and (can be ordered through your butcher as) number 185C in the NAMP book
, the butcher's bible."
Just made one this evening all the way out here in Chicago. The Tri-Tip was delivered via Roadfooder JRPfeff; cooked using Meathead's method: http://www.amazingribs.co...ia_tri-tip_steaks.html
Other than misjudging the quirks of my own grill, it worked perfectly. Those quirks produced a medium-medium well steak instead of the desired medium rare, but still incredibly juicy, tender, and flavorful. I have one more Tri-Tip to cook and I'll make the necessary adjustments to achieve the medium rare I'm after.