The eleleg macro calculates hip foot and knee positions for a pair of legs. When on the ground the leg is always straight so the hip positions must move up and down. The two legs are calculated together because the hip position is also dependent on the width of the pelvis/shoulders because the width of these will remain the same but rotate as the creature walks.
Bisection is used to calculate the angle of the pelvis/shoulders to the horizontal at any point during the step cycle.
dinosaur
pig
dragon