I think Justine's is more of a weapon even though Mary is right.
She does hit off the back foot a lot, i thought i was the only one that noticed that, BUT she has soooo much of her hand BEHIND the racket that it becomes more offensive when she connects.
Roger on the other hand rolls his wrist and uses his to set up his forehand more.
Roger has more topspin, Justine drives hers more.
What trips me out is hwo she can be still drive the ball so deep from behind the baseline to the extent she overpowers and drives two handers back.
Serena is the only 2 hander she couldnt drive backwards too much.
Serena ran it down and seemed to rob Justine of preparation time.
Maria hasnt been able to do that yet for some reason, which speaks to a lack of great movement.