The moment it fails
The copy from the phone or the camera was interrupted, and now every player reports an error. If that recording exists nowhere else, the error message feels final.
Why players refuse it
MP4 and MOV keep an index, the moov box, that says where each frame is. Many cameras write it last, so an interrupted copy loses it even when the frames arrived. MKV, AVI and MPEG-TS spread more structure through the stream, and a player may stop at the damage.
What happens to the file
The tool looks for frames in the raw data, reads or infers the codec, picture size and timing, and writes a new index. Damaged passages are cut out, the video starts on a key frame and the stated duration matches what was found. Frames are not re-encoded.
What you come away with
A video that opens and contains the intact part. This is the case where the tool does best, but the missing part cannot be created. If settings cannot be inferred, a healthy video from the same camera can be given in Advanced options.
Common questions
Will the video be complete?
It will hold what is intact. A missing part cannot be recreated.
Is the video re-encoded?
No. Intact frames stay as they are; only the container is rewritten.
Sources
Documents consulted for this guide.