Cricket: England v South Africa

In regards to Hawk eye and Lbw's i don't get it when it say's the ball is only half hitting the stump's, if it's been called out then it backs the call but if it's been called not out and challanged then it stay's not out? Surely it's either in or out either way, not both depending on who challanged the call and why!

If you are going to trust hawkeye, then it should be out if the ball is hitting the stumps at all, but because of the margin for error, they have deemed it needs to be more than 1/2 the ball.

I think this system is working quite well, it gives teams a chance when umpires get it wrong, it's not supposed to be a tool to try and get wickets where there are none. And as there are only two, they need to use them wisely.