Here's two things I've found while you're waiting for Brian to answer.
First, ANY FORWARD hand effort on the downswing will cause slack. This can sneakily creep into the swing of even those who know better, because it's seems to be an innate tendency of humans, AND, this incorrect tendency gets reinforced by most 'Top 50' type instructors, who teach "Shaking hands with the target", "Throwing clubs at the green", and "Throwing folding chairs across a basketball court". As Homer says, "If you don't know that you hit down, assume you didn't".
The 2nd guard against slack is, of course, Extensor Action.