This is important! To maximize tenderness, a tri tip roast (like all meat) must be cut across the grain, meaning perpendicular to the grain. The tri tip is a little trickier to cut because the grain runs one way on two thirds of the meat and another way on the rest. Look at the long muscle fibers of the meat when it's cooked and let your eyes guide you. Here's an excellent visual for how to cut a tri tip roast.