the cochlear region of your inner ear helps maintain balance; for one thing, there are perceptory hairs in that region which are in a fluid-filled cavity that allow you to sense direction (this is how, for instance, you can close your eyes and know which way is up in a swimming pool).
Sudden violence to that region, like concussive force, will drop you quickly. When I was in the Marines, I saw a guy standing on the sponson boxes (located in the side of the hull) outside an M60A1 tank when it fired a 105mm round out the main gun; the concussion wave hitting his ear dropped him as though the main circuit breaker in his brain blew.
On a more pertinent note: the concussive impact of a cupped palm-slap will drop a guy much bigger than you quickly. One guy I read about who was a bouncer in British nightclubs for many years was a big fan of slaps, as they can be delivered in such a way that an unwary loudmouth won't see them coming and drop them, sometimes, sending them stumbling over their bodies and putting them off balance as well. After that, they're too wobbly to fight, confused, and even if they figure out what happened, nobody wants to admit to getting bitch-slapped out cold by another dude in a bar.
On a more personal note: I got suckered with one of those almost 20 years ago when I was in the Corps, taking a late-late-late night stroll outside my barracks (couldn't sleep) whereupon I got jumped by three guys (I was totally not expecting three well-dressed dudes to try mugging me on base, of all places). I got caught with one of those to my ear, and it rocked the ever-loving shit out of me.
I learned that night about the effectiveness of shots like that. And the importance of being alert and aware at all times, especially when dealing with multiple strangers at 3 AM. And why it's a bad idea to turn and fight three guys who jumped you can who you were about to get away from (read: because getting your ass kicked even harder sucks worse). All kinds of stuff.
