In These Cases - Citizen Eco-Drive 200 Meter BM6400-00E User Manual

G. IN THESE CASES
Two second
interval movement
12
[If the watch warns that the energy is running short]
The second hand starts moving at two second intervals
in order to warn that the watch will stop functioning after
about 1 week. (Insufficient Charge Warning Function)
In such a case, expose your watch to light for a while to
cancel the warning. (The second hand returns to moving
at one second intervals when the energy is recharged.)
If the watch is left short of energy, it will stop functioning
after 4 days or so.