Keeping the blue dot in front of you (toward you), strike the blue dot with a big open hand. Line the volleyball and tossing hand up with the serving/striking shoulder. The ball is tossed or delivered in front of the striking hand. The tossing hand delivers the ball just inches from the point of release, bringing the tossing hand high. Flip the tossing hand around and you will have a blocking hand in place. The first POP is striking the ball and the second POP is the hand or sound of the tossing hand contact. It acts as a backstop after the ball is released.
Pop-pops will make two sounds when done correctly. Beginners may hit the ball at the same time—one big POP! Just keep working on the pop-pop rhythm that starts with the striking hand making the initial contact.