Cooking the shrimp in the shell prevents the tasty juices from escaping.
Grate half the ginger. Finely chop the remaining ginger. In a large bowl, mix all the ginger, the lime juice, and cumin; season with salt. Toss with the shrimp until coated. Cover and let marinate in the refrigerator for about 2 hours.
Thread 5 shrimp onto each of 4 skewers. (If using wooden skewers, soak them in hot water for at least 30 minutes to prevent burning.) In a large skillet or grill pan, heat the oil over medium. Cook the shrimp until just cooked through, 2 to 3 minutes per side.