Double boot (also known as cold double boot, double cold boot, double POST, power-on auto reboot, or fake boot) is a feature of the BIOS and may occur after changes to the BIOS settings or the system's configuration, or a power failure while the system was in certain sleep modes.
Changing some parameters in the BIOS will cause this issue, even for just setting a system as stock speeds, such as initializing the current CPU and memory clocks. When certain parameters, such as the ones before, from the BIOS are set, it will require a reboot.
If the computer didn't have any power and had just been plugged in these parameters will need to get implemented again, and since these parameters require a reboot, the computer will do a quick reset to implement these parameters that are set in the BIOS. Even after the computer is turned off, these parameters won't need to be re-entered for as long as the power supply is still receiving power.
A double boot proceeds as such, the PC will power on for about two seconds, then off for about a second, then it will turn back on and display the POST screen and continue to boot up normally.