StickyKeys is an accessibility feature to help Windows users who have physical disabilities, but it is also used by others as a means to reduce repetitive strain injury (or a syndrome called the Emacs Pinky). It essentially serializes keystrokes instead of pressing multiple keys at a time: StickyKeys allows the user to press and release a modifier key, such as Shift, Ctrl, Alt, or the Windows key, and have it remain active until any other key is pressed.
Microsoft first introduced StickyKeys with Windows 95. The feature is also used in later versions of Windows.
To enable this shortcut, the Shift key must be pressed 5 times in short succession.
This feature can also be turned on and off via the Accessibility icon in the Windows Control Panel.
To turn off once enabled, just simply press 3 or more of the Sticky Keys (Ctrl, Alt, Shift, Windows Button) at the same time.
Over the years, this feature has posed difficulties for users who naturally use the Shift key heavily, such as gamers. When a user presses the Shift key 5 times within a certain period of time, the shortcut activation popup will be placed above all other applications. This can be fixed by going into the control panel and disabling the window. 
Sticky keys makes an alert sound on Windows computers and laptops, but on Mac or Apple computers, it makes a silent tapping sound. On Mac, sticky keys is pressed only once on the shift key.
- Fingerblaster 9000, The. "TF2 Sticky Key/Keyboard Glitch?". Steam Forums. Retrieved 2013-11-30.