Phones do not rotate their sensor when you rotate your hand. They record the same way up regardless and write a rotation flag into the file, trusting players to spin it at playback. Your phone's gallery respects the flag; plenty of browsers, projectors and editing tools do not. Same file, two orientations.
Flippy fixes it at the pixel level: each frame is transposed and the video is re-encoded once at quality matched to the source. After that, no player has to guess.
One rotation pass is visually lossless. Just avoid spinning the same file back and forth repeatedly: every re-encode of a lossy codec shaves a little, so get the direction right and keep that copy.