If you use a screen saver, it will start automatically when your Mac is not active for some time. You can set a shortcut to start the screen saver when the pointer to the corner of the screen.
1. On a Mac, choose Apple menu> "System Preferences," click "Desktop & Screen Saver", then click the "Screen Saver."
2. Click the "firing angle."
3. Tap the angle you want to use the pop-up menu, select "Start screen saver" and then click "OK." 
If you want to combine Control, Option, Shift or Command key to start using the firing angle screen saver, when the pop-up menu to open the key is pressed. For example, if you press the Shift key, then the "Start screen saver" command becomes "Shift- start the screen saver."
When the pointer over the screen angle setting, the screen saver will start. To stop the screen saver and return to the desktop, press any key, move the mouse or trackpad.