Bad teqnique, instead of two fists they should lay their hands on top of each other with palms facing up otherwise their forearms arnt rotated outwards so theyre presenting the sides of their arms as the platform instead of the flat undersides of their arms. And the impact point shoukd be the bottom of the forearm, just above the wrist, not the fists themselves. Knee bend and posture is pretty good though, although they should be extending during the contact to add power and direct the ball.