This move can feel like it kills your back everytime you do it, but once your back becomes more flexible you will notice it doesnt hurt anymore. You dont need to bend your back so its horizontal to the ground, just have it enough to balance the ball.
First practise putting the ball on your chest, where you feel the most comfortable, raise your arms and balance it. The ball shouldnt be moving. Another way to ease the pain from your back is to have one foot on its toes, so if you balance the ball on the left side of your chest raise your right foot on its toes and have it slightly infront of the left foot. Visa versa.
Once you can balance it, practise by throwing the ball onto your chest. You shouldnt throw it high, throw it to your chest softly. As the ball comes close to your chest you move it back into the stalling position. As you would when you catch a tennis ball; as the ball goes to your hand you move it back. Keep practising this till you can do it perfect.
The last step is to practise it from footstall. You dont want to throw the ball high with your foot, you want to throw it towards your body at chest height, if you do it right you should be able to stall it how you did when you threw it with your hands, same principle. Once you can do this you can also try from a juggle.
Once you can stall perfect, try doing it without raising your arms, this will improve your upper style alot. :thumbsup:
NOTE: Dont stress your back too much, if your back begins to hurt, take a break, you definatly do NOT want to injure your back.
Goodluck :thumbsup: