How does Canon 7D realize the precise combination of fixed-point autofocus in portrait photography?

Canon 7D chooses fixed-point autofocus to achieve more accurate focusing. In portrait photography, if you want to use fixed-point autofocus to achieve more accurate focusing, you should manually select the autofocus point after framing and composition. Generally, the autofocus point should be aimed at the eyes. The operation method is as follows. Don't focus first and then compose.