How to create a "Turn Off Screen" Action?

I recently moved to Elementary OS Luna. I especially use the Synapse Indicator launcher, because I like to work with my keyboard for everything. In Synapse and in the Synapse Indicator (and I believe, generally, in the Gnome dekstop and I'll dare saying in all Linux DEs) there are "actions". Among actions there is *Shut Down*, *Restart*, *Lock Screen* etc. which can be run with Synapse and probably too from the command line. I would like to create an action for *Turn Off Screen* (which is different from *Lock Screen*). How can I create this action and be able to launch it from Synapse?