About Rippling and Overwriting the Exposure in the Timeline View
By default, you can only use your mouse to drag and drop frames in the Timeline view. However, if you zoom in sufficiently, you are able to tweak the exposure of each drawing by clicking and dragging the junction between two drawings.
Just like in some video editing software, when tweaking the duration of a drawing by dragging its side, you can decide whether to ripple or overwrite subsequent frames. This is determined by whether you grab the edge of the drawing by the left or the right side of said edge.
Although not required, this functionality is especially useful when displaying thumbnails in the Timeline view, as you can preview the exposed drawings as you modify their exposure. As a reference, tweaking exposure in the timeline is only possible when you zoom in on the timeline at least close enough to fully view the thumbnails of drawings exposed on two frames. For more information, see Displaying Drawing Thumbnails in the Timeline View.