How video compression actually works
Keyframes, bitrate and why cutting before compressing is free quality.
Most frames are not stored
A video codec stores a complete picture only occasionally — that is a keyframe. Everything between keyframes is stored as the difference from its neighbours, which is why a static shot compresses to almost nothing while a shaky handheld pan does not.
Why fast cuts land on a keyframe
To start playing at an arbitrary second, a player needs a complete picture to build from. If we cut without re-encoding, the clip has to begin at a keyframe — so the cut may land up to a few seconds from where you asked. Re-encoding lets us build a new keyframe exactly where you want it, which is precisely the fast-versus-precise trade-off.