The Push to Bottom of Stack tool pushes the selected path to the lowest z-level in the model. Besides an x-y location on the page, all paths have a z-dimension that tells whether it is above or below other paths. To push a path to the bottom of the stack simply touch one of its snapping points. If you have trouble selecting the desired path, you may have the snapping Header Option Button button set to Snapping to Top Path rather than Snapping to Bottom Path or vice-versa. See Snapping for more information. Here’s an example: