try "throwing" the top up with the string after it passes the halfway point. Raise both hand up and towards each other a little so the tension is reduced and the string lets go of the tip.
You can also try doing it as slow as possible, with as little tension as possible, raising the hand the top leaves slowly and continuously, lowering the target hand as the top approaches. This doesn't look as good as a straight wire walker but will help you learn the ropes. Pun intended.