The "two slappies" are a fun way to help teach young players to cover as much surface area on the ball as possible, but with a big hand and not a stiff hand. Top spin will come from a ball positioned out in front of the player's hitting shoulder and the ball should always stay in front of the player.
The "10 deliveries" part teaches a steady and well-positioned toss while adding the contact and watching for the ball to turn forward. This is the ball coming off of the hand with top spin.
Tip: Young players should reach or strike the ball with a high-hand and finish with a driving hand feeling; one that has the striking hand headed toward the top of the volleyball (or the wall/net).