Found via reddit, excerpt:

Choking happens when we let anxious thoughts distract us or when we start trying to consciously control motor skills best left on autopilot. ...

In her new book, Choke: What the Secrets of the Brain Reveal About Success and Failure at Work and at Play, Beilock deconstructs high-stakes moments—the ones seen around the world and the ones only our mothers care about—to explore why we sometimes falter, and why other times we nail it. ...

What goes wrong in our brain when this happens? 
Working memory, housed in the prefrontal cortex, is what allows us to do calculations in our head and reason through a problem. Unfortunately, it’s a limited resource. If we’re doing an activity that requires a lot of cognitive horsepower, such as responding to an on-the-spot question, and at the same time we’re worrying about screwing up, then suddenly we don’t have the brainpower we need.

Also, once we feel stressed, we often try to control what we’re doing in order to ensure success. So if we’re doing a task that normally operates largely outside of conscious awareness, such as an easy golf swing, what screws us up is the impulse to think about and control our actions. Suddenly we’re too attentive to what we’re doing, and all the training that has improved our motor skills is for naught, since our conscious attention is essentially hijacking motor memory. ...

How can I prevent myself from overthinking? 
You might think that writing about your worries would just make them more salient. But there is work in clinical psychology showing that writing helps limit ruminative thoughts—those negative thoughts that are very hard to shake and that seem to grow the more you dwell on them. The idea is that you cognitively outsource your worries to the page. Writing about worries for 10 minutes right before taking a standardized test is really beneficial.