How to Make It
Preheat grill to high heat. Why? Thin cuts cook quickly; high heat will give the exterior a good sear quickly without overcooking the meat.
Slice 1 breast half horizontally, cutting to, but not through, the other side. Open halves like a book, laying chicken flat. Repeat with remaining chicken. How? Place one hand flat on top of the breast to hold it steady. Hold knife parallel to cutting board with other hand, and make a long, smooth cut horizontally through the middle of the breast. Stop about 1/2 inch short of cutting all the way through.
Brush oil evenly over chicken. Combine sugar and remaining ingredients; sprinkle evenly over both sides of chicken.
Why? You've doubled the surface area of the meat--meaning more room for the spice rub and more area to pick up chargrilled flavor.
Grill chicken 2 to 3 minutes on each side or until done. Why? Because the butterflied breast is half the thickness now, it will cook very quickly.