A phrase commonly adopted by CrossFit boxes is "no mirrors, no machines, no mercy." Most people understand why "no machines." But a lot of people don't get why most CrossFit boxes don't have mirrors. The short answer often given is CrossFit trains for real life, and there are no mirrors in real life. But UB not having mirrors is a conscious choice with considered reasoning behind it.
Your body mechanics are important in any exercise. We especially talk a lot about your spine's position during movements. And when we say "spine," we mean your entire spine - from skull to tailbone. If you are looking to a mirror to check your form, there is a good chance your craning your neck to see it. This probably means your not in an ideal position for the lift, defeating the purpose for looking.
How many times have you, while lifting, had at least 3 cues in your head. Now, imagine there is a mirror in front of you. This adds another list of elements to your lifting experience, all of which are considered distractions. For example, in the Olympic lifts, it is important to focus on a single point ahead of you to establish equilibrium. Finding one spot to focus on is already difficult to do while people are walking in front of your field of vision. Now place a mirror in front of you. There are a thousand more distractions, and holding one focus point will be more difficult because of as you move, the objects' reflections change as your angle of vision changes.
The Mirror Lies
Now imagine again focusing on those cues while the mirror is there. You will naturally default to correcting based on what you see instead of what you feel. Instead of attempting to feel complete hip extension, you might change mid-lift based on how it looks instead. This is wrong for a couple reasons. First, if you watch a lift from the front view, you are often missing a lot of the intricacies of the movement's mechanics. This is why the coaches usually watch a lifter from a 3/4 angle or side view. Second, you could be fixing something that looks visually wrong, but is not the source of the problem. Usually a list of faults in a movement originate from one essential fault. You need to learn how a movement feels and develop a strong sense body awareness in order to replicate it. This will not happen if you rely on your reflection.
Learn from Film
If you really need to see what is going on with a movement whether you are learning or experienced, ask a fellow athlete or a coach to film it. This gives you the opportunity to focus on the feel, and then check to see if what felt right or wrong was in fact right or wrong, and from the correct angles to view the movement from. Nobody around? Put your phone on a stand like this one, or this magnetic one, and do it yourself. Have other filming recommendations? Post to comments!