The ruling on the LBW- Once the umpire provides his inputs on a particular delivery, it becomes dead. i.e. the state of play stops. Applying the same rule in the above LBW case- if the umpire gives it not out, then he doesnt really effect a decision via the moving of his hands. The ball as such is still in a state of play. However, if he gives it out (irrespective of whether the decision is challenged or not), the state of play stops. I believe this is where the rule emanates from + the challenges in trying to change the rule.