For example when I record a rig walking and then I'd like to make it walk backward so how can I do that without much effort
You can flip the animation around by actually inverting your animation data in your animation editor.
- Go to your dope sheet editor.
- Set the current frame to the beginning of the animation (probably frame one)
- Select the frames you want to reverse. Press S and type -1
- Move the inverted animation back to where it is supposed to be positioned (using G)
- Play back the animation!
When you're scaling the frames by negative one, you are really just reversing the animation. Similar to turning a shirt inside-out.
If you don't want to reverse F-Curve key-frames, the NLA editor has a reverse option you can enable non-destructively per-strip. The NLA editor allows you to work with your animations in a non-linear manner; this allows you to repeat, scale, and reverse them, among other things.
In the NLA editor, press N to open the sidebar, then enable Reversed in Active Strip → Playback Settings.
Another way to do this is to render your animation, then go to the video sequence editor. Add your rendered images or movie, press N and go to the options in the filter section and check the option to play the active clip backwards. Go back to VSE press control F12 (to sequence render, again), with the backwards box checked and this will render your animation in reverse.