Can you place a Ping Pong Ball in the center of a cup or bowl and not have it move to the side? Answer is yes, if you break down the surface tension. Check it out!
Placing the Ping-Pong ball at the surface of the water creates curvature in the water surface all the way around the ball. The surface tension will act to minimize the total surface area. Unless the ball is placed exactly in the middle of the cup
Which is nearly impossible!
There is an asymmetry in the surface curvature that will draw the ball toward the side., nearest to the ball, thus minimizing the total surface area.
When the soap is added the surface tension is reduced and the path to minimum surface area is less steep. As such, the ball will drift much more slowly such that it appears to be stable in the center of the cup.