Photo: What does the referee do to Kolarov?

Two participants in the second round derby in the English Premier League between Liverpool and Manchester City were captured in an unusual position.

The main referee on the match that was played on Sunday on "Enfield Road", Andre Marriner and Vladimir Kolarov - Serbian international player are "owners" of the "photo of the day".
Marriner was right behind Kolarov when Kolarov was preparing for a free kick for his team. Marriner placed his hands on the back of Kolarov, what he meant with this move remains unclear?