Wash, peel and cut the yams into pieces. Soak in boiling hot water and remove immediately.
Heat the oil in a wok over medium heat to about 250 degrees F, or until small bubbles appear around a piece of green leaf tossed into the oil.
Add the yams and deep-fry until they are cooked or golden brown in color. Pour all but 2 tbsp of the oil out of the wok. Turn the heat to low and add the sugar, stirring constantly until it dissolves.
Cook until the syrup caramelizes and turns light brown. Immediately add the yams and remove wok from the heat. Quickly toss the yam pieces so that they are well coated with the syrup, which will spin into a thread. Place on a plate and serve immediately.