To use it you must choose a clip with NO B-frames (that is a video encoding terminology). If you try to use it on a such clip you will be blocked by a dialog asking you to use Properties > Convert on it.

After that, it is basically Filters > Time > Time Remap > Keyframes. Go to this forum post for more information.

Other Fixes

Fixed a possible crash using a PNG with alpha channel (transparency) especially with a Size, Position & Rotate filter (broke in version 20.06).
Fixed changing Properties > Video > Color Range does not work (broke in v21.01).
Fixed selecting text with the mouse in various filter numeric fields (broke in v21.01).
Fixed difficult to enter some numbers in various filter numeric fields (broke in v21.01).
Fixed unable to make tracks as short as before (broke in v21.01).
Fixed keyframes disappear when changing selected clips on various filters (broke in v21.01):
- Gain/Volume
- Pan
- Pitch
- Saturation
Fixed appending to the timeline may change the current track.
Fixed Properties > Extract sub-clip on a file with cover art.
Fixed File > Export Frame on a clip with a Text: Rich filter.
Upgraded JACK audio library on Windows to version 1.9.17.