To prep the swiss chard, cut off the stems and chop the leaves into 1-cm (1/2-inch) pieces. Chop the leaves into 4-cm (1.5-inch) pieces. Separate the stems and leaves.
Heat oil in a cast iron pan (or wok, or a heavy skillet) until hot. Add garlic. Stir a few times until fragrant.
Add the chard stems. Cook and stir a few times until they start to soften.
Add the leaves. Quickly stir a few times. Add sugar and swirl in soy sauce. Immediately stir to mix well. Cook and stir until the chard turns tender. Transfer to a serving plate immediately to stop cooking.