In the last panel, Hobbes’s paws are still suspended in midair. Calvin has kicked the board right out from under them.Based on where Calvin is standing and the direction that the board is going, in real life you would have to use your right foot to kick the board in that direction from under Hobbes’s paws. But it seems like Bill Watterson has drawn Calvin’s left foot doing the kicking so that he looks better to the reader, sort of like the way that actors on a stage try to keep facing toward the audience as much as possible.If Calvin were kicking with his right foot, Watterson would have to show Calvin’s body from the side with his right foot crossed over in front of his left one. Then we would be seeing his face from the side instead of the front, limiting the expressions that Watterson could draw on Calvin’s face.The shadow under Calvin indicates that he has kicked with such force that his body has gone straight up off the floor, a la cartoon physics. It’s like the way that Calvin’s wagon bounces along above the ground, or his dad’s car rises up off the pavement as it goes over hills.