If crab was cooked the day before, rinse before using. Rinse shells.
Pulse garlic and ginger into a paste in a food processor.
Set a roasting pan over two burners on high heat; add oil. When oil is hot, add garlic-ginger paste and cook, stirring, until golden, 30 seconds to 1 minute. Add Singapore Chili Sauce and heat, stirring with a wide spatula, until boiling. Boil until reduced by half, about 15 minutes. Add crab and heat until warmed through, stirring often, about 5 minutes.
Serve crab in roasting pan, or arrange in a deep platter. Top with the back shells, if you like, and cilantro. Serve with rice.