To kill crabs humanely, place them in freezer for two hours.
Bring large pot of water to a boil over high heat. When crabs are no longer moving, plunge them into boiling water for one minute, or just until shells begin to turn red. Transfer crabs to baking tray to cool.
Remove legs and claws from crabs. Using back of knife, lightly tap body shell to crack it. Remove top shell from each crab body and clean insides with your fingers. Cut main body of each crab into four pieces.
To cook crabs and serve:
Place large wok over high heat. When hot, add oil to wok. Add shallots, garlic, chilli, and ginger and stir 30 seconds, or until fragrant. Add crab pieces, including claws, and cook, stirring frequently, for five minutes. Add rice wine and cook, stirring frequently, for three minutes, or until wine is reduced by half.
In small bowl, stir vinegar, soy sauce, sweet chilli, and sugar and drizzle over crabs. Cook, tossing frequently, for two minutes, or until crab meat is cooked through.
Transfer crab pieces to serving platter and spoon sauce over. Garnish with coriander and serve.