Sometimes there is a need to switch boot devices on a pc (for example to start a recovery DVD / CD with a WinPE / WindowsPE, a live Linux or another Recovery system disk or USB stick.
On most PC’s and Laptops you can do this with pressing either the F8 or often the F5, F12 or DEL key while the PC starts.
Normally this opens up a boot menu where you’re able to switch to a different boot device.
On modern pc’s you will be able to boot most WinPE disks, but not linux or some other live cd until you disable the boot security in the bios options.
The UEFI boot process is blocking many unsigned operating systems in standard configuration, to prevent viruses from installing rootkits.
But don’t forget to reenable the Boot security after, or you open the door to viruses and rootkits.