An action held open and repeated runs on, and somewhere inside that stretch an unplanned new state arrives unnoticed.
框 · Frame
[subj] [verb]著[same verb]著，就 [new state] 了
觸 · Trigger
An act runs on and on, and partway through, a new state creeps in that nobody steered toward.
序 · The move
1Take an action that can be held open and run on (read, walk, talk, listen).Can this act be sustained over a stretch, not just done in one quick go?
2Double it with 著 on each half, V著V著, to stretch it across time.著 rides on each verb half, marking the act as held and ongoing, not finished.
3Close with 就 [new state] 了 — a state that arrived during the stretch, unplanned.Did the second state slip in WHILE the first act ran, unnoticed? If it was a deliberate next step, this frame is wrong.
例 · Examples
1弟弟常常在床上看書，看著看著reading on and on (the act held open and running)就and right then, with no gap睡著了fell asleep — the new state took hold。
My little brother often reads in bed, and while reading on and on he drifts off to sleep.
一…就… fires the second event the instant the first lands, a clean reflex with no stretch (一下課就回家). V著V著…就…了 needs the first act to run on for a span, and the new state drifts in somewhere inside that span. 一 marks a trigger point; V著V著 marks a duration the change leaks into.
V著 (durative)
Plain V著 holds one action open as a backdrop (戴著帽子). Doubling it, V著V著, sets that backdrop running long enough for a turn to arrive; the …就…了 tail is what makes it a drift into a new state, not just a held one.
V了一V
V了一V shrinks the action to one brief, light pass (笑了笑). V著V著 does the opposite — it stretches the action out so a slow, unnoticed change can settle in during it.
✗ 看就睡著了 (no stretch, reads as prompt sequence) → ✓ 看著看著就睡著了
✗ 看著看著就回家了 (a steered next step, not a drift) → ✓ 看著看著就睡著了 (an unplanned state that slipped in)
✗ 看著看著就睡著 (the change-of-state 了 dropped) → ✓ 看著看著就睡著了
English 'while reading, he fell asleep' uses one 'while' clause; learners render it with 在 or a single 著 and lose the doubled stretch, so the slow drift into the new state goes unmarked. The doubling is what carries 'on and on until, without noticing'.