With the lid open?
You can't turn it off if that is what you mean. You can turn the brightness down all the way but that won't mimic the effect of it being off as in clamshell mode.
To enter clamshell mode :
Connect external mouse and keyboard via bluetooth or USB.
sleep machine with power source and display connected
wake machine with keystroke or mouse movement.
Enjoy.
There are more friendly tips and tricks in the manual.