"Chunking" describes the process by which a complex sequence of individual steps — each one demanding real attention the first several times — gradually collapses into a single automatic unit that runs with minimal conscious oversight. This is the mechanism that makes habits so efficient: instead of consciously managing dozens of small decisions every time, the brain can trigger the entire sequence with a single cue and free up mental capacity for something else entirely.
The same efficiency that makes chunking so useful is also what makes bad habits hard to interrupt in the moment — once a sequence has been chunked, it runs largely below the threshold of conscious monitoring, which means noticing and interrupting it requires deliberately re-engaging attention that the brain has specifically learned it no longer needs to spend there.