Hi Michael,
The Short Answer is:  Your inverter charger on your solar system senses voltage drop and tells the generator to start via the Deep Sea Two Wire Auto Start Controller (which is included within the generator).
More Detail:  Auto start generators work on a two wire principle. In this case the Dunlite and Gentech 8kVA Auto Start Honda Models, they are fitted with a Deep Sea Two Wire controller. This controller relies on a signal from your inverter charger (as part of your solar system) to join "Two Wires" to close the circuit and start the generator and vice versa open the circuit and stop the generator.
Your inverter charger on your off-grid solar installation has the smarts that detect voltage drop. You can program the inverter charger to signal the generator to start when the batteries reach this pre-determined voltage. The inverter charger can also be programmed to accept a frequency range and voltage range that the generator puts out.
Hope that helps - Regards, Paul