I've been catching, preparing and eating fish for years, so goodness knows why I haven't come across this trick before.
Basically, it's a quick and easy way to remove the bones and leave the flesh and skin intact, ready for cooking.
First, gut the fish and trim off the head, tail and fins. Now put your knife down. You don't need it for the next bit - honest! Place the fish stomach-down on the chopping board with the tail end towards you and the head end away. Spread out the flaps you made when you slit the stomach, so it stands up on its own.
Now the clever bit. With your hands on either side, use your thumbs to press down hard on the backbone. You'll feel it give, as if you have squished the fish flat. Actually this is the backbone coming away from the flesh. Repeat all down the backbone.
Then pick it up, find the head-end of the backbone, and pull it away from the flesh. It will come away easily, taking most of the bones with it.
You can then fold the fish back together, cook it flat, or cut the two halves apart, depending on how you want to cook it.
I tried the technique with herrings, and it worked brilliantly. I wondered if it would be possible to slice through the flesh at the head, but leave the backbone intact. That could make it even easier to remove the backbone in one piece. I ran out of herrings to experiment with, but I'll try it next time.