This is a skill game where you have to throw a boomerang at your teammate. You know what boomerangs do: at a certain moment they make a U-turn and then fly back your way. So it's important that your teammate catches the boomerang either on the 'way out', or on the 'way back'! Mind the time: it's limited in each level. The faster you are, the more points you'll earn. Try to catch the bonuses (rotating stars) when you throw.
First choose the direction and click to set it. Then you choose the curve the boomerang will make while flying to your teammate. If you want to change the direction or curve, press the spacebar.