[Gatts]

When your spell is maxed (lvl 3) then holding:
CTRL : LEVEL 1
SHIFT: LEVEL 2

If your spell is just level 2, than any of shift or ctrl means level 1.

You can check it when your mouse cursor is over that spel, you can press and hold any of CTRL of SHIFT and the spell will change accordingly (mostly you will see that mana cost is lower, if not than the effects are weaker)