Glad you got your head round the concept - took me a while too to be honest!
If you're thinking about buying them, i'd have a good long think about whether you really need them - have a look at their pros and cons. Most people will feel the whoopie sling is all win win, but i personally think its more of a faff and not a simpler set up.
I do use whoopie slings now, but sometimes i wish i could just tie my hammock to the tree with a once around and a simple bow knot and all done! Instead i find myself faffing with whoopie slings, toggles, marlin hitch, straps and dutch clips. The only thing that makes me continue with them, is the flexibility in choosing my trees. I can now use trees much further apart than i could with the DD straps - but this again isnt always a positive in a thick wood.
I suppose im basically saying, personally, i think its a different suspension method, rather than a better method. So before you spend money on some gear (relatively inexpensive i suppose) have a good hard think about it. My mates dont use them, and they never have any problems.