I noticed in German these keys don't seem to have a description in the options menu but just name the function(?).
Fixed for next version: All English descriptions have German translations. When I wrote the options menu for D Lix in late 2015, I was too lazy to translate all the descripionts, causing the game to display a fallback string.
In the options menu it says that fast rewind is 1 sec and fast frameskip forward is 10 sec but if I do one fast forward then I can undo that with about 4 fast rewinds, which doesn't seem to fit the time description. (Do I have a thinking error here?)
I have further the feeling that there are sometimes odd (bigger) jumps with fast rewind but that's hard to assess since it makes jumps by design anyways there.
The backwards timeskip is only 15 physics updates (= 1 second at normal speed) when your game runs at 60 frames per second. To compensate for the expensive recalculations, at lower framerates, I increase the number of rewound physics updates per skip. I haven't documented that anywhere.
From your recent screen recordings, it looks like your machine runs Lix at 20 fps by default? That's very low, sadly, even though you still get reasonable physics.
After you know this explanation now, do you consider it a bug or a feature to increase the rewound physics updates per skip at low fps?
I like how it goes faster the longer you hold the hotkey, but it sure is confusing without explantion.
-- Simon