The open face also increases the loft of the wedge for a high soft shot that won't roll very much once it hits the green. He has slightly lighter than normal grip pressure to help him use a little more hand action for this shot. The ball is positioned slightly forward of where it would normally be so that Tiger's clubhead can enter the sand behind the ball. Since the ball sits slightly on an upslope, Tiger has set his shoulders so that his right is slightly lower than his left, matching the incline of the bunker. Tiger has done a great job of maintaining the knee flex he established at address, creating a stable base to support the rotation of his upper body.
Tiger lets the club work down into the sand behind the ball through the hitting area.
Tiger maintains his knee flex to make sure that he hits the sand first and not the ball. What's important to note at the end of the swing is the position of the clubface. Notice how the face of Tiger's sand wedge is pointing to the sky as he holds his finish position. He holds the clubface open by not allowing his right forearm to cross over his left through the hitting area - which causes the clubface to close, reducing loft and trajectory. His finish is very low, yet the sand and ball is propelled high up into the sky.