From a recent Psychological Science,
In everyday life, individuals typically approach desired stimuli by stepping forward and avoid aversive stimuli by stepping backward... Cognitive functioning was gauged by means of a Stroop task immediately after a participant stepped in one direction... Stepping backward significantly enhanced cognitive performance compared to stepping forward or sideways. Considering the effect size, backward locomotion appears to be a very powerful trigger to mobilize cognitive resources.
As Chris Chatham notes,
This work is remarkable not only for demonstrating how a very concrete and simple bodily experience can influence even the highest levels of cognitive processing (in this case, the so-called "cognitive control" processes that enable focused attention), but also because performance on the Stroop task is notoriously difficult to improve.
When you suddenly realize that a task is more difficult than you assumed it would be, or when you face a particularly difficult choice in pursuit of rationality, you may find it useful to literally take a step back. For those of us who are particularly interested in making good decisions, this may also serve the purpose of self-signaling, as Yvain and commenters discussed earlier.
Chris's post has a link to a pdf of the paper.