The Fast Startup feature in Windows, enabled by default, allows the kernel session to remain hibernated during shutdown, which results in quicker boot times by restoring a saved state. In contrast, selecting Restart clears the kernel session entirely, performing a complete boot of Windows. The process of shutting down with Fast Startup involves closing applications, logging off, keeping the kernel loaded in memory, saving the kernel and drivers to hiberfil.sys, and powering off, while Restart closes applications, logs off, ends the kernel session, and reboots immediately without saving a kernel snapshot. Fast Startup can lead to persistent issues from previous sessions, making Restart a better option for troubleshooting. To perform a full shutdown bypassing Fast Startup, users can execute the command "shutdown /s /t 0." Microsoft advises against permanently disabling Fast Startup, as it enhances shutdown-to-boot transitions.