Hi Graynomad,
The 1hz signal is for timing purposes. I use a DS3231 on the main module to act as a timekeeper. It's SQW output is programmed to operate at 1Hz. This timing signal is used to calculate average power consumption, for example. When the power module senses the rise in the SQW signal, it stops calculating power consumption and squirts the results to the main module. Then the main module notes the power data and writes it to disk, along with a time stamp from the ds3231.
One of my projects ahead will be to change this setup to record on a second by second basis but then to only squirt summaries every 10 seconds. The reason being that this would allow the main module larger time windows to do time-intensive tasks like GPRS connections. Also, it gives all other modules on the chain more time to respond to data requests.