This menu item is available from the Clip Jobs menu that appears when you The Project Bin on a clip in the Project Bin or from under the Project Menu menu when a clip is selected in the Project Bin. It is used to create a clip which plays in reverse.
This feature became available in version 0.9.6 of Kdenlive.
When you select theoption from the menu, a new clip is created in the Project Bin. It has the same file name as the clip from which it was created, but with a .mlt extension appended. You can then add this clip to the timeline and when you play it, the video of the original source clip will played, but in reverse.
1=According to legacy Mantis bug tracker ID 2933 some clips will only produce white image and error on reverse.