Cast Pressed Spell Upon Pressing Another

What exactly does it do? I read in forums that it's intended functionality, step by step, is to hold down 1 ability then while holding that one down, press another and your held down ability will go off. So thinking that this would be fun to try for characters with combos like Vel'Koz I enabled it and no matter what I did I couldn't get the first ability to cast when switching to the second ability. It'd just switch without casting so instead of necroing an old thread for answers I'm creating a new one. Anyone got some info on how to get that to actually work?