Not related to regular spinach but rather to cactus and purslane (order Caryophyllales (Carnations)), this plant has a similar flavor to spinach, but milder without so much oxalic acid. The leaves are thick, almost succulent. One cultivar, "Rubra", has red stems.
While regular spinach is a cool temperate plant which doesn't like the tropics at all, Malabar Spinach is a tropical vine. A fast growing perennial, it is harvested continuously by cutting new growth. It can be grown as an annual in warmer temperate regions.
More on Carnations.
This plant is used throughout Southeast Asia and southern China in stir fries and, particularly in Vietnam, in soups. It also appears in tropical African cuisine as a leafy vegetable.
Buying: Malabar Spinach is now fairly common in Asian markets in California (I get it from the Hawaii Supermarket in Alhambra) and can sometimes be found at farmer's markets.
Storing: This plant isn't very durable, particularly since the leaves tend to be damaged when it's bagged for sale. Keep it refrigerated and use it within 2 days.
Yield: 1 pound from a typical bag yielded 13 ounces when all stems larger than 1/4 inch were discarded for a yield of 81%.
Cooking: For stir fries and the like cook as for regular spinach. in just a little oil. Free water on the leaves from washing is sufficient to get it cooking. Stir frequently and stop cooking as soon as the leaves are limp and of a uniform cooked color. Do not overcook or it will become slimy and leave a metallic aftertaste.
In India Malabar Spinach is used as a thickener similar to the way okra is, so it's cooked much longer in curries, sambars and the like, and with a lot more other ingredients.
Nutrition: Malabar Spinach is high in Vitamin A and C as well as Iron and Calcium. It is low in calories but has a good protein to calories ratio, and plenty of soluble fiber.