This is true even for a "straight punch". In the animation of a steam engine below, watch, sound off and imagine that the piston is a punching fist and the push rod is an arm, etc. Do you see linear or
circular motion?
Now in a Wing Chun straight punch, the elbow doesn't make a complete circle coming up and over like in the animation. but it does circle ...a very small circle which recycles a lot of the energy. Hard to see but easy to feel!
Check out the following clip (sound off) from about 3:15 - 3:25. Watch the elbows! See the circles?