It is a combination of lots of things but mainly practise.
I wont go in to loads of detail on how I do it because some bright spark who can't spin at all will tell you I'm giving out the wrong info.
Jamie is amazin' at his spins. Best thing to do is grab someone who can multiple spin at a weekender and ask for tips as they are very hard to explain over the net.
A few vids that might help
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