She is too cool for Yoga but just right for hot pants! This is from Week #136/300.
This is one of those shots with a pleasing depth-of-field: just enough in-focus and enough out-of-focus to make everything tie properly and highlight your subject just right. And yet - you can see the focus is way up vertically on her eyes and not in the center on her belly. This is really only possible by using a selective single point auto focus method.
By default the camera's auto-focus system tries to make a best guess of what you want to focus on and it sometimes gets it right. The focus points are generally gathered around the center so it is much easier for you to take a picture with good sharpness and focus if your subject is in the center. However, we know that that doesn't always work out. You want to place your subjects to the sides for creative or other reasons and then if you want to shoot a full portrait then you generally want the focus point right up at the top of your frame.
If you want to take pictures with shallow depth of field (generally anything at or lower that f/2.8) and yet control where exactly the focus is then you need to stop the camera from deciding on its own and take matters in your own hands using a selective single point auto focus.
Almost all cameras give you the ability to manually select the auto-focus point. The two photos on the right show focus point selection buttons from two popular Canon models - the Rebels and the xxD and the pro models.
A somewhat acceptable way to achieve the same result is to press the shutter half-way, and recompose the shot while keeping the shutter-button half-pressed. While that works fine for deeper depths of fields, it will not work for close-ups or shallow depths of field.
And what is the drawback? Well, you need to keep changing the damn thing with almost every shot. That is yet another thing to keep in mind. If you don't keep selecting it or pushing it back to default - you will actually get worse results. Initially you will find yourself fiddling with the system and losing cool moments. However, eventually it will become a habit. I use it for almost every short and I just do it without even thinking about it.
It is a small price to pay to get good, sharp, well-composed images, specially portraits.
Here is a good article from DPS that provides more details on how this feature can be enabled and configured.