Push vs. Pull
When stepping forward from a front stance, should you push forward with the back leg or pulling forward with the front leg? Or, should you do a combination of both? Let us try an experiment.
Face a wall in a front stance and push against the wall with your arms. Lift the front leg. Notice that the forward thrust is essentially the same. Now, lift the back leg and push. The forward thrust is greatly reduced. In fact, you may find yourself falling backward unless the weight of the body is shifted forward. This illustrates that the back leg's push is more important the any pull of the front leg.
Next, stand away from the wall in a front stance and step forward with the rear foot. Notice how much you pull with the front leg while stepping. If you practice this for a while, you can see that active involvement of the muscles of the front leg can provide a definite improvement in the speed of forward movement of the body during stepping. Notice how the forward motion of the rear leg helps pull the body forward once it passes the torso while at the same time the front leg begins pushing the body forward.
One of the primary determining factors in figuring out the relationship between pushing and pulling is the distribution of weight forward or backward. Try the second experiment again with the weight further back than usual and then further forward to see its effect on the pushing and pulling. The further forward the weight is situated, the more the front foot plays in moving the body forward. While the weight is behind the front foot, this can be considered a pull. After the weight passes the front foot, the function shifts to that of push.
Also affecting the contribution of the back leg's push is the amount of flex in the leg. If the back leg is straight, very little force can be applied by the leg, since the only forward thrust comes from the ankle and foot, unless the leg is momentarily bent before the forward movement, which of course takes time and may telegraph your movement to the opponent. If the leg is slightly bent, then more forward thrust may be generated. Taekwondo America stresses having a straight back leg while in a front stance.