Okay I hit my first (clip) spinning ducking tricks yesterday so I'm no expert but I'm still on a high which is why I have to reply

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Requirements:
-a good ducking set (height and tightness)
-a good spinning set (straight upness and no sideways movement of the body, so you have an accurate idea of where the bag will be after the spin)
The height required for the set is pretty straightforward. I set like I would for a duck and then 'just' backspin. You can also set slightly higher to make it somewhat easier and get the feel of spinning ducking. This makes the duck less tight but I think that's okay for starters (this is what I did).
Also you need a little speed (though there's no need to rush) on the spin, because you spot the bag when it is still on its way up, and duck more or less when it peaks.
Hope this both helps and makes sense!