Each A53 is idled by taking them to the WFI state. This is done through Trusted Firmware-A (TF-A). For idling CPU, the PMU firmware raises TTC interrupt (timer 9) to TF-A, which issues software interrupt to each alive A53 core. The respective cores then clears the pending SGI on itself and put itself into WFI.
The last core just before going into WFI issues pm_system_shutdown (PMU firmware API) to PMU firmware, which then performs APU only restart flow.
This feature must be enabled in TF-A for recovery to work properly. It can be enabled by building TF-A with ZYNQMP_WARM_RESTART=1 flag.
Modifying Recovery Scheme
When ENABLE_RECOVERY is turned on, AMD provides a recovery implementation in which a FPD WDT timeout results in the invocation of APU subsystem restart. You can easily modify the recovery behavior by modifying the code. Alternatively, an example of PMU firmware invoking system-reset on FPD WDT timeout is detailed in AMD Answer: 69423.