I can personally attest to the effectiveness of an open-hand slap to the side of the head.
About a year ago, a former in-law came banging on my front door one night. I stepped out onto the porch to see what he wanted. His father and my sister had been through a messy divorce a few months before. I don't really know what his problem was with me, but he launched into a rant calling me every name in the book. I suggested he leave before I called the law on him. Instead, he took a wild looping swing at me.
Now, I am not a fighter and definitely do not have fast reflexes. This guy is 35 yrs. old, 5'11", 185 lbs. I'm 49, 5'10", 142 lbs. in soaking wet clothes.
Anyway, I slipped his punch and came up with an open palm right smack on the guy's ear. His eyes rolled back in his head and he was out like a light even before he tumbled down five steps and landed flat on his back in my driveway. I swear to God he was snoring like a lumberjack!
All I could do was look at my open hand and say, "Did I do that?"
When he woke up a few minutes later, I threw his sorry ass in his pick-up and told him he'd better think twice before he tried any such crap again...he hasn't, and I don't really care if he can still hear out of that ear.
And I went back in the house with an ear-to-ear grin on my face.