Soak the tamarind in 1/4 cup of hot water for 10 minutes. Strain and reserve the liquid. Crack the crab shells but do not remove them. Soak them in the tamarind liquid for an hour. Grind the red chilies and ginger together to make a paste. Set aside. Remove the crab claws from the tamarind liquid and reserve the marinade. Heat oil in a pan and fry the cumin seeds, ground ginger-chili paste and garlic until an aroma comes from it. Add the onions and fry until golden. Then add the crab claws, brown sugar, salt and the tamarind marinade. Mix well. Cover the pan and simmer for 10-15 minutes or until the sauce has thickened. Serve hot.