The procedure varies according to the bootloader:
- Chameleon with graphical interface: as soon as the countdown appears you have to press any key but return. At that point you select the system you'd like to boot. After doing that, you can type the desired boot flag (e.g. -v). What you type will appear in the lower-left corner of the screen. To boot press return.
- Darwin bootloader / Chameleon without graphical interface: as soon as the countdown appears press F8 to stop it. Select the system you want to boot using the arrow keys. Type the boot flag and press return.