I have had to enable this feature on tablets at work to enable bypassing of the Ctrl-Alt-Del screen without connecting a keyboard. In our case (Acer Iconia) we had to hold a special Windows key on the bottom of the tablet and press the power button simultaneously.
As the article linked to above says: not every device has a designated "security button".
If you don't have a full keyboard on your whiteboard, then I would assume it has some sort of security button to compensate- as with tablets. You can probably work it out by trial and error: enable the security button feature, then log out and go through all the buttons until something unlocks it - or a combination of something + power unlocks it.
I know this 3-key combination is a security feature but it's worth keeping in mind that you can disable it very easily - causing it to go straight to the password prompt screen instead.