overhead hitting

In basic swing, the purpose of arm moving up from behind will cause our forearm open up from back. So allowed us to hit the shuttle fall on the top of our head or fall back of our head.
This is very important because we do not need to run behind the shuttle to get enough strength to hit the shuttle.
At today coaching session, the reason most people can get enough strength to hit the shuttle when the shuttle fall over their head because arm move forward first and elbow bend forward before hitting the shuttle.
If the basic is correct and still not get the power to do clearing. This is a way that some of my student get to understand it. In the basic swing, i just as my student use all his strength to push his hand up, hand made a sudden stop at the hitting point and hand must be straight.