The A-B Looper repeats a chosen section of a YouTube video over and over. You set a start point (A) and an end point (B), and the player loops everything in between — so instead of dragging the scrubber back every few seconds, you can play the same bar, phrase, or move as many times as you need. YouTube's own loop option repeats the whole video; this loops just the part you care about.
Musicians loop a single lick to work out fingering or timing. Dancers loop an eight-count to learn choreography move by move. Language learners loop one sentence to shadow a native speaker's pronunciation. The trick is to keep loops short and, while you're learning, slow them down using YouTube's playback-speed control (the gear icon → Playback speed) at 0.5× or 0.75×, then bring the speed back up once it's reliable.
For the full method — picking loop lengths, why repetition beats re-watching, and tips for each type of practice — read our guide: How to loop a section of a YouTube video for practice.