Place the crabs in the freezer for about one hour to put them to “sleep”. To clean the crabs, lift the flap on the underside and pull off the top shell. Remove and discard the grey, spongy, finger-like gills. Rinse the crabs quickly under cold water to get rid of any gunk. Use a heavy knife or cleaver to cut each crab in half lengthwise, then cut each half into thirds. Use the back of the knife to crack the claws and legs to let the sauce seep in while they cook. Place the tomato sauce, sugar, fish sauce, tamarind, rice wine and 60ml (¼ cup) water in a bowl. Stir to combine. Heat the oil in a large wok over high heat. Add the garlic and chilli and fry for about 30 seconds. Add the crab pieces and give everything a bit of a mix. Add the tomato sauce mixture and toss the crabs around to coat them well. Cover with a lid, reduce the heat to low and cook for about five minutes. Remove lid and shake crabs around, then replace lid and cook for another five minutes or until the shells turn orange and the meat is cooked through. Pile the crabs and sauce into a large serving bowl and scatter over the coriander. Serve with lots of napkins and small bowls of water with lemon wedges to clean sticky fingers.