In nature, bending of living structures occurs due to differential growth, with more on the backside/outside of the curve. This suggests that the signal for light-related bending must cause more growth on the backside/less lit side of the stem or coleoptile.
Work Step by Step
Think about curves--there is more on the outside, just as your tires travel more on the outside edge when you turn your car. The signal involved in light-related bending must promote this in a stem.