It stops as soon as you either finish the race or leave the server.
You can find it in the main menu under "Demo".
Its in the "auto" directory.
Or if you finished the race you can find it in the "auto/race" directory
To save it you can click on "Demos directory" and move the demo file into the parent folder "demos"(edited)
DDNet client has a multitude of commands and settings that can be bound to nearly any key or mouse button. You can enter commands into the console, which by default is accessed by pressing F1.
DDraceNetwork, a cooperative racing mod of Teeworlds - Do not overwrite predicteded CharacterCore with Snap Data · ddnet/ddnet@1dacb33
19:57
My guess is that the glitches only happend on very slow PCs or with very very good internet. Because for me they only happen on local servers, with old DDNet version