I don't know about you, but when my swing focus is either the ball, my shadow, my backswing arm position, or my hands, I almost always hit an errant shot. The reason is that I have incorrectly shifted my focus from a smooth swing towards my starting line to something else.
The left side of the golf swing equation includes setup/alignment, the body parts (and their properly sequenced movements), the ball, the swing plane (and resulting path of the club), and it culminates on the starting line (i.e., target). The right side of the equation contains a swing that has had the ball merely get in the way, usually with a good result--the ball traveling close to the intended target with the intended ball flight. A proper focus in golf should always be the target and intent towards that target (e.g., high, low, draw, fade, short, long, etc.).
Too much emphasis on the ball or hands programs the mind to release the club prematurely, causing fat shots, while a release well past the ball comes from proper focus. In almost every case, what happens is that the body stops moving and the arms and hands take over, either because of a focus on the hands or too much focus on the ball. The mind is unconsciously doing what you've told it do. A smooth swing through the ball towards a target is replaced with a hit at the ball. And a hit usually produces crap, even if it's not fat. The focus wasn't the target; the focus became an intermediate point in the swing--a middle part of the equation.
I hate playing midday on a sunny day, because my shadow can be so distracting. I can easily become fixated on watching how my shadow looks, seeing errors in my swing. In this case, the ball gets lost in the equation, because the eyes shift away from it. And the attention is definitely not on the target or intended ball flight; it's now on the errors I perceive in my shadow or how I appear in the swing. Glancing towards the position of the arms on the backswing is equally destructive in the same way. In this case, it's important to refocus attention. The eyes must see the ball, and the focus must be "out there." Using shadows and looking at arm positions can be great on the practice tee as a drill, but these actions have no place on the golf course.
The mind is an amazing targeting system. This is why we're told--when chipping--to focus on a landing zone on the green while we're taking practice strokes. It's also why we're told to focus on the starting line or break point of a putt instead of the cup; it's why we practice putting while looking at the breakpoint and not the cup (so that we get a good feel for the distance). If we focus on the flag when chipping, we're likely to land the ball near the pin and watch the ball roll far away or even off the green; we unconsciously program the mind to put the ball where we focus. The same magic that allows us to usually land the ball on the green where we focus is the same magic that allows us to make all sorts of errors when our focus erroneously shifts towards the wrong parts of the equation. And when it does we're usually dead in the water.
The next time you blow a shot, ask yourself what your focus was immediately before the swing. In almost all cases, I bet the right side of the equation was deemphasized in your mind in favor of the left side. You were focused on the means to and end and not the end itself.