Because the PS4 keeps a backup of the previous firmware as a failsafe, tools like can be used to swap these slots, effectively "reverting" the console to its prior state. Key Tools for Downgrading
How to Revert the PS4 to a Previous Firmware (Full Tutorial) ps4 tool downgrade v100 new
A hardware setup used to glitch the console’s Syscon chip into a debug state for firmware writing. Because the PS4 keeps a backup of the