Weights deliver an extraneous load to the body which means that they become an immutable force that doesn't lessen as the muscles fatigue. It then triggers the adaptation response which leads to increase in strength and muscle size. Muscle Factory is a workout that cleverly uses this principle to challenge a number of upper body muscle groups. Use your breathing when lifting to tighten your abs and also engage your core a little more and you've got yourself the kind of workout you feel you've done the day after.
Extra Credit: Keep the plank throughout during up and down planks.