When would I apply this technique?

If your opponent attempts to grab your leg for a takedown, commonly called a single leg or double leg takedown. Or when an opponent attempts to tackle you at your waist or legs.

What is my goal with this technique?

Your goal is to stay on top and not stop their advance, but guide their advance on you downward. Imagine your body as an upside down ramp, you are guiding the force of an opponents advance on you downward. The bigger or heavier your opponent, the more space will be needed to guide them downward.  

Common mistakes with this technique:

-people tend to use their hands to try to stop an opponents advance, your hands and arms are much weaker than an opponents entire body which is barrelling toward you. Instead imagine your hips as your hands, use your hips to guide them downward.

-people tend to bend at the waist, this allows your opponent to grab your legs and take you off balance. Instead make sure your legs stay behind you, this may seem awkward, but it will keep your legs out of reach.

-people tend to let their shoulders and head lean forward, this disperses your weight on your opponent. Instead make sure you look up and your shoulders are high. Your back should be curved away from your opponent, this will focus all your weight on your hips, making it hard for your opponent to grab onto you and take you down.