Tue 09-Oct-12 01:10 AM | edited Tue 09-Oct-12 01:12 AM by Ferguson
Are you asking how to get the buttons to do this, or if it will work reliably?
To answer the latter part -- it depends. If you are shooting a very fast lens, with very shallow depth of field, this actually may NOT work. But in most cases it will.
I was asked about this before and couldn't convey it well in words so drew this picture. On the left is the initial compose and focus on the left hand person. The right is the exact same focus shifted to the center - same camera position, just recompose.
Notice how the focus is now behind all three people.
Whether they are in focus depends on the DOF and their depth; but it always happens if you recompose after focusing.