Freeze beef 20 minutes or until easy to slice. Trim fat from steak; cut into 1/4-inch thick slices. Place slices in resealable plastic bag, in shallow baking dish.
In small bowl stir together soy sauce, brown sugar, vinegar, 1 tablespoon water, garlic, ginger and red pepper flakes. Pour over steak in bag; seal bag. Marinate in refrigerator 2 to 12 hours, turning bag occasionally.
Coat grill rack with nonstick cooking spray. Preheat grill to medium direct heat. Drain steak strips; discard marinade. Thread steak onto skewers, folding steak back and forth.** Grill skewers uncovered 6 to 8 minutes or to desired doneness, turning skewers once halfway through grilling.
* Marinate time extra
**If using wooden skewers, soak for 30 minutes before grilling to prevent burning.
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