The reverse footlock acts as an ingenious way to both have complete control over your entire leg, and individual control over the smaller elements such as the toe, ball or ankle. This is done really by an understanding of parenting and the parent / child relationship.
By creating the joint in the heel first before the subsequent ones that run along the Toe, Ball and Ankle, you are ensuring that all of those joints will ultimately follow the heel joint as they are the “children” and they will follow the parent in the hierarchy.
Furthermore, when you then parent the IK handles of each actual foot joint (be it toe, ankle or ball) to those reverse footlock joints, then wherever those RF joints are pulled, the IK handle follows. This means that while each individual RF joint has control over one joint’s movement, at the same time they all are moved by the Reverse Foot joint in the heel. It results in a sturdy, efficient and diverse method of moving the legs.
Of course too much manipulation or not following the process correctly can often cause major problems – but one think I have learnt about rigging is that you have to follow the processes methodically. A slip of concentration can mean an entire rebuild. But when the rigging is finished having followed all of the steps, it is reliable and exceptional at animation.
A note on Pole Vectors:
Pole Vectors control the way a point is facing without having to alter the joints (in the case of the leg Rig, the direction of the Knee
