"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.
Duhigg walks through everything technically involved in backing a car out of a driveway: unlocking the door, adjusting the seat and mirrors, checking for obstacles, calculating distances, coordinating the brake and accelerator — a sequence that would be genuinely overwhelming if a person had to consciously reason through each step every single time, the way they did during their first few driving lessons. Once the sequence becomes habitual, all of it happens with what Duhigg describes as "hardly any thought," freeing enough mental capacity that a driver can simultaneously remember a child forgot their lunchbox inside.