So what does the stuff after "except" mean? The ball did rebound from the keeper, therefore the rest is negated, surely?
Yes, that's the problem here. In fact I would have thought that the second part (after "except") would be relatively common, so probably quite often invalidates the first bit (by the words as written anyway). Whether they intended that interpretation is another matter of course.
In this case the keeper did bring the ball "under control" (as defined by the first part) but only after also fulfilling the requirements of the second part. Which kind of negates it? Needs rewriting to express more clearly what they meant. And those two parts of the rule don't belong in the same sentence!
