Well, it's just a trick you need to get the feeling for. Start one foot on the break, and other on the clutch. Let clutch go halfway, without stalling the car, and quickly move your right foot from brake to gas. Press on gas pedal, while releasing clutch. If you do it right, the car starts driving forward, even on a upwards hill. It takes practice, and every car feels different.

If you're on a really steep incline, you'll have to press both the brake and gas pedal at the same time using your right foot, while feathering the clutch with your left. I've heard this called the "heel toe" technique.

If your engine has enough torque or if the hill isn't steep enough, you can ignore this and just ease off the clutch while transitioning from the brake to gas.