Many photographers do this. They want to take Subject A, but A is somehow hard to lock focus on(perhaps due to low contrast). So, photograhers focus at Object B, an object which is the same distance as Subject A from the photographer. So, after locking focus, they recompose back to A, and take the shot. At the end, a nicely well-focused photo.
"Not so," says Michael Hohner. He gives his explaination of his statement here: http://www.mhohner.de/recompose.php. However, after reading it a couple of times, I still dun get it. How can A be still in focus when you recompose back to B? Is he wrong?