To perform an unblockable combo attack you need to press
3 times with certain timing with a Jedi or Sith character.
After the first
pause briefly then press
again (The lightsaber will glow with stars on this attack to show you did it right) pause briefly again and press
(The lightsaber will glow with stars again and you character will perform a special move)
Here's a video to show the combo in action.