Triple-press on the home button can be used to invoke different features. I use it to invert colours at night to make the screen darker.
Here is how you do:
Go to Settings > General > Accessibility > Accessibility Shortcut.
Tap Invert Colours to activate it on triple-pressing the home button. This is indicated by a checkmark to the right.
Triple-press the home button to activate inverted colours.
Inverted colours aren't particularly good for watching movies but it works well for reading.