If you've used a motion path to make something like a shape travel in a curve or line, you might've noticed that there's a certain force in the way the shape travels, accelerating at the beginning of the path and decelerating at the end. This is because of Smooth start and Smooth end—options that are on by default.
The options are available when you select Effect Options from the drop-down menu on the effect in the task pane list. If you turn them off, the shape moves with more of a floating quality.
Auto-reverse is another option available for a motion path effect. It sends a shape forward along the path to the end, then backward along the same path to the start. This automatically doubles the duration of the effect. If you used the Repeat options on the Timing tab, also in the Effect Options dialog box, you could keep an item going back and forth—specifying that it stop after a certain number of repeats, on a mouse click, or at the end of the slide.