Scrubbing

The word is borrowed from tape. To find an edit point on a reel to reel machine an engineer disengaged the capstan and rocked the reels by hand, dragging the tape back and forth across the playback head until the sound of the splice arrived. The gesture was called scrubbing, the audio it produced was a smear rather than playback, and the whole point was that the operator heard something at every position on the way. Digital editors kept the name, kept the smear (Audacity still plays audio while you scrub, at whatever speed your hand moves), and video kept the visual half: frames going past under the finger.

Scrubbing and seeking get used as if they were the same act, and the difference is the feedback in between. A seek is a jump: press a point on the track, arrive there, and nothing is shown between where you were and where you landed. Scrubbing is a held drag where every intermediate position is rendered, which is what makes it a search rather than a guess. That is the whole reason a reader drags instead of clicking. They do not know the timestamp they want, they know the shot, and the only way to find it is to be shown the frames while looking.

The engineering cost lands on that promise. Decoding a frame at every pointer position is expensive, so players cheat in well understood ways: a sprite sheet of low resolution thumbnails generated at upload and shown in a bubble above the pointer, a preview that snaps to the nearest keyframe, or a seek that only fires on release while the drag itself is served entirely from the thumbnail sheet. Choosing to preview but not seek is usually the right trade on a network, and it is why a scrubbed video often looks like it is moving before it audibly is.

Precision is the other half, and it is a Fitts problem: two hours of footage across three hundred pixels is roughly twenty four seconds per pixel. Editors answer with a zoomable timeline, players answer with fine scrubbing (drag away from the track and the ratio of pointer travel to time drops, an iOS convention), and both answer with keyboard steps in seconds rather than percentages. The control being dragged is the scrubber; scrubbing is what the reader does to it, and a scrubber that previews nothing has been reduced to a seek bar.