This CLI debugging interface grants the attacker full access to the computer’s hard drive, despite the presence of BitLocker. The reason is that during the Windows 10 update procedure, the OS disables BitLocker while the Windows PE (Preinstallation Environment) installs a new image of the main Windows 10 operating system.

Source: Holding Shift + F10 During Windows 10 Updates Opens Root CLI, Bypasses BitLocker – Slashdot