NVIDIA tells us that the sole point of the LP Cortex A9 is to
provide lower power operation when your device is in active standby (e.g. screen is off but the device is actively downloading new emails, tweets, FB updates, etc... as they come in). The LP core runs at a lower voltage than the GP cores and can only clock at up to 500MHz. As long as the performance state requested by the OS/apps isn't higher than a predetermined threshold, the LP core will service those needs. Even with your display on it's possible for the LP core to be active, so long as the performance state requested by the OS/apps isn't too high.
Once it crosses that threshold however, the LP core is power gated and state is moved over to the array of GP cores.
[...]
The vSMP/companion core architecture is
a unique solution to the problem of increasing SoC performance while improving battery life.