If successful, the phone will switch from Emergency mode to Flash mode. Then flash the FFU:
In the context of Lumia devices, "emergency files" usually refer to (Flash File Updates) or specific drivers required to resurrect a "bricked" phone. When a Lumia 650 enters an emergency download state, the standard Windows Device Recovery Tool often fails to recognize it. In this state, the phone’s bootloader is damaged, and it requires a manual intervention—specifically, a "dead USB" flash—to write the operating system back onto the storage chip. lumia 650 emergency files free
Pros