Answer
If you stand on the swing rather than sit, the time for a back and forth swing is shortened.
Work Step by Step
Some internet research about pendulums, or referring to Footnote 2 on page 358, shows that the period $T = 2 \pi \sqrt{\frac{L}{g}}$.
When you stand on the swing, the pendulum's effective length L is reduced because you have moved the pendulum bob (your body) closer to the pivot. This decreases T, the period.
This is discussed on page 358.
