It happened in the second set as Stepanek approached the net and hit a poor volley to Djokovic. As Stepanek had no chance of stopping a forehand slam by Djokovic, he used the unorthodox strategy to try to get Djokovic to whiff.
It didn't work, as Djokovic still made the shot and ended up winning the match in straight sets. However, Stepanek did make the match more interesting and gave the fans their money's worth.
Video via AustralianOpenTV. H/T Busted Racquet.