Ah I wanted to talk about this the other day but completely forgot. I honestly think that this is a combination of perception and some science. The cornflour itself won't change the relative densities of the meat, nor the salt content, so is unlikely to directly influence the juiciness. This is where brining becomes vital - osmosis takes place and if done correctly, will turn any piece of meat into a juicy, tender delight. However going back to the cornflour - what I possit is happening in this method is that the meat is absorbing the cornflour far deeper than merely coating it. This has two outcomes - the meat fries far crispier which means that when you bite into it, there is a contrasting perception of crisp and juicy (minimal though.) The second thing is that it will create a layer around the meat that is far thicker and it will do so quickly, meaning that it will trap in the moist centre of the meat and steam it in its own juices.
There is method to my madness - I had known about this method for a while but had never tried it. I was however skeptical about cornflour being able to denature proteins in meat, so I used a chicken breast, cut into four pieces to test this - two pieces were placed in a cornflour marinade for a few hours. The other two were left as they were. I deep fried one piece of cornflour marinaded chicken and one piece of cornflour dipped (a few seconds before frying) chicken. Without a shadow of a doubt, the cornflour marinade made it far crispier and delicious. So I baked the other two pieces in the same oven using the same method. However on this occasion I found no difference at all in the relative juiciness or tenderness. So my conclusion (albeit not a double-blind scientific one) is that the juiciness and tenderness can be attributed to perception and a little science, but not for the expected reasons.
In short, it does work...
