This app uses the Device Administrator permission for locking the phone.
Lock your phone by closing the flap. No need to press the lock button any more. If you don't have a flap cover, then merely putting the mobile into your pocket will lock the screen. Or else simply wave your hand over the proximity sensor of your phone.
Also you can specify to disable the app while a call is in progress. Time can be set for which the app will remain disabled once you unlock the screen. This avoids unwanted locking.
By switching off the screen you save the battery also.
An ad free, permission friendly version of F2L is available for purchase @ https://play.google.com/store/apps/details?id=com.easwareapps.f2lflap2lock_adfree