The key trick to using an impact driver of any kind is that you must “preload” it by rotating the driver in the screwhead in the loosening direction and firmly holding it there and THEN giving the driver a smart cuff with a dead blow hammer. The axial force of the hammer does not translate into very much rotation of the tip. The tip rotates juuuust enough to crack the screw loose (I'd estimate perhaps 5-10 degrees tops depending on how hard you hit the end of the impact driver). Thus, you need the "preload" to ensure that the tiny rotation of the driver tip actually turns the screw and is not simply taken up in the sloppy fit between the drive tip and the screwhead.
Using this technique, I have found that even the tightest screws with totally messed-up heads will pop loose on the first, or at most the second, whack.
If you simply insert the driver into the screwhead and start wailing on it with a hammer - you are likely to either strip the screwhead or bust it clean off the screw (leading to tears and/or non-approved language).
The other tip I can offer based on several decades of using impact drivers is that, because they don’t get used very often, the little ramps inside the driver mechanism can get “crusty” which results in the force of the hammer blow NOT being effectively transformed into screwdriver tip rotation. For that reason, I give my impact drivers a little squirt of oil just about every time I use them to make sure that everything inside is nice and slidy.
Pete
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the drivers with a few tweaks. One bugs me a little though. It's what they call 
firmly into the screw head, reduce pressing force, prevent driver tip damage, and help prevent cam-out. Applications include construction, automotive, industrial use, general fastening, and more.
