Start with a Sleeper.

Using your throwhand pull either to the right or left and give just a slight tug as you do so. The yo-yo as it returns grabs the string using its spin and actually tightens and loosens the string the same as a UFO would. (Tighter if the hand moves towards your throwhand side and looser if it moves towards your non-throwhand side.) Catch the yo-yo as it returns to your hand.