I gotta say, I was pretty skeptical about this(still am).
Then today out on our kayaks, with no fish around, I started knocking on my hull and a small school (maybe a few fish) started feeding on the surface right between my wife and I.
It was gone in an instant, the wind kept us from staying over the fish, and we didn't catch any. But it would be a strange coincidence, that being the only surface activity we saw all day.
I told my wife that I needed to install some speakers and a stereo in my kayak now so she can catch more fish. She told me to keep knocking.
We'll see.