It only goes through friendly creeps.
Friends of Blitzcrank step aside when he does his thing.
Even if you are wedged in between your own minions, a well placed Rocket Grab can still get you. I've done it before, and of course had times where it only grabbed a minion.
If there's a clear path between minions wide enough to shoot that hand through, you'll get grabbed. Also, the shot follows a path from the right edge of Blitzcrank, not his center.
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