Not possible unless you push the crank or move the wheel. While riding the upper loop will always have the tension because hat's how it pulls the sprocket around.

If you hold a fixed wheel bike stationary, you can transfer tension between the upper and lower loop by pushing the pedal back and forth. Usually the pedal will have about 1/4-1/2" of this "backlash" movement. On a freewheel bike it's impossible to tension the lower loop because the freewheel will always feed chain into it.