When you hibernate, all RAM is transfered to storage before your computer shuts down. Next time you start your computer, it is restored to the exact state it was in. This takes a while(3-7 minutes), so you'll see it say "preparing to hibernate".
I recommend you not use hibernate too often. It's not good for security or stability. You can disable it in power options (start, run, powercfg.cpl). [ theymos's advice column | Ask theymos A Question ]
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