Put the chicken in a large bowl and cover with the tamarind pulp. Add the soy sauce and garlic and mix well. Best if refrigerated overnight, but can also marinade for 30 minutes before grilling.
Prepare the grill fire for indirect grilling. Remove the chicken from the marinade and pat it dry.
In a small bowl, combine the coriander, chili powder, and a pinch of salt. Dust the thighs on both sides with the spice rub.
Cook on the hot part of the grill until grill marks form on the bottom, 3 to 5 minutes. Turn and mark the other side, about 2 minutes more. Move the chicken to the cooler side of the grill and grill until cooked through, 7 to 10 minutes.
Let the chicken rest on a cutting board for 5 minutes and then cut the meat into strips and serve.