OS X Yosemite: Use hot corners to start your screen saver
If you use a screen saver, it starts automatically after your Mac is inactive for a time. You can set a shortcut so your screen saver also starts when you move the pointer to a corner of the screen.
Choose Apple menu > System Preferences, click Desktop & Screen Saver, then click Screen Saver.
Click Hot Corners.
Click the pop-up menu for the corner you want to use, choose Start Screen Saver, then click OK.
If you want to start the screen saver by pressing the Control (^), Option (⌥), Shift (⇧), or Command (⌘) key together with a hot corner, press the key while the pop-up menu is open. For example, if you press the Shift key, the Start Screen Saver command changes to ⇧-Start Screen Saver.
When you move the pointer to the corner of the screen you set, the screen saver starts. To stop the screen saver and return to the desktop, press any key, move the mouse, or touch the trackpad.
If you want more security when you stop the screen saver, see Require a password after waking your Mac.