Use abilities on keypress - Abilities or items in your action bars are used when its key is pressed, rather than when it is released.
This section provides the action bar settings available in-game. Once at the Settings menu, click 'Interface', then 'Action Bars'. You can get there by pressing the escape key or clicking 'Main Menu' from your menu bar at the bottom of your screen. First, navigate to the Options menu and select 'Settings'.