The heart of the system is a smart battery combiner that handles connecting / disconnecting the batteries automatically.
When the engine starts, it allows the engine to charge the cranking battery first, then links the batteries together in parallel so they can both be charged by the alternator.
Once it sees the voltage drop below ~12.7v for a minute, it will separate the batteries. Your accessories connected to the bus bars continue to run from the second battery and won't drain your cranking battery.
