Clean and separate the crab into pieces: rinse well under cold water, carefully removing the yellow gills or spongy insides. Twist apart the legs and claws. With the blunt side of the cleaver, crack the legs and larger front claws and set aside. (This makes the crab easier to eat after it’s cooked.)
In a small pot, bring 2 cups of water to a boil. Remove from heat and stir in salt.
Heat oil in the wok over medium-high heat. Stir-fry ginger and scallions until aromatic, about 1 minutes. Add crab pieces and stir-fry for another minute. Add salted water, then cover the wok and let the crab steam for 10 minutes. Transfer to serving dish.