Originally Posted by
YouKnowWho
When you use 'double legs", you want to use the top of your head to hit on your opponent's belly. Your head will be down at that moment.
When you shoot in with head up, if your opponent pushes your fore-head back, your body forward momentum and your opponent's backward fore-head pushing will put a lot of pressure on your neck. No matter how strong you may be, your neck is always a weak spot in your body.
If you can manage to make your opponent's arms "not to be free", head up or head down won't make any difference.