I would say refer to my post with the Vince Carter picture for a deeper explanation on that front. All one-handed dunks are not created equal. I'm not sure who can dunk on this message board and who can't, but I think even if one has just goofed around on a lowered bucket, it'd probably be relatively easy to notice what happens to one's off arm when dunking perpendicular to the rack (as opposed to slamming with body directly facing the rim).
To be completely honest, you probably don't even need a rim to mimic this action. I honestly just gave it a shot while walking down the hallway. If I mimic a slam while facing the basket, off-hand won't be in front. But if I tilt my jump a la Blake and vintage breakaway alley-oop catching Vince, the off-arm's naturally arising.
Here is what that jumping like that looks like without a defender:
http://www.youtube.com/watch?v=PnVC_t2ufR4#t=0m16s
To further illustrate, here's an example three different types of Russell Westbrook dunks. And surely, I wouldn't expect three examples from one dunker to fully convince you. But I find these to be good indicators:
1. One hand, one foot (thus no arm lead):
http://www.youtube.com/watch?v=aq15yGbop7k#t=0m32s
2. One hand, two feet, squared (off-arm raised but not leading):
http://www.youtube.com/watch?v=zzpaSeIbpLQ
3. One hand, two feet, leaning (off-arm leads by nature):
http://www.youtube.com/watch?v=tY-pw-Qs9vw#t=0m16s
To be clear, this doesn't mean Griffin has never been able to simultaneously clear someone out with an off-arm. I'm pretty sure we've all been guilty of it at some point. My stance is more or less that the arm in that specific dunk motion, is coming up no matter what. It's not a matter of a player consciously saying, "You know what? I'm going to lift up my arm and beat someone with it."
The biggest difference I've seen between Blake's dunks and many normal dunkers, is he drops a heavy hammer.