In a small bowl, mix together the brown sugar, paprika, thyme, salt, black pepper, onion powder, garlic powder, cayenne, and cumin.
Rub beef with the liquid smoke to coat, then rub with the spice mixture.
Lightly coat a 6-quart oval slow cooker with nonstick cooking spray. Pour in the barbecue and Worcestershire sauce; stir together. Add beef and coat with the sauce.
Cover and cook on low until fork tender, for 8-10 hours.
Transfer beef to a cutting board; let rest, loosely covered with foil, about 20 minutes.
Cut meat across the grain into 1/2-inch slices and serve with the sauce.