Notice steps in the form. Are they performed with smoothness, like water flowing? Or does the foot clunk down. Imagine that the foot is at the edge of a cliff. A smooth slide step out leading with the heel will let the body know what lies ahead, such as the edge of a cliff. A foot that opens out and steps down without the slide will lead the body off the cliff. Also, the slide step that is lead with the edge of the heal is actually a low kick. This same step could be completed with more space between the foot and the ground to become a kick leveled to an opponent.