The Russians must have the most efficient military in the world to mobilize and act so quickly to changing events like that.
Actually, tensions were high prior to the military actions. The week prior Russia publicly warned Georgia not to invade. They also recommended that Ossetians evacuate the capital which they mostly did. So, the invasion came as no surprise. In all likelihood, the Russians also had satellite & human intelligence to give them a heads up. Troops massing on the border would not have gone unseen. When the Georgian peacekeepers abandoned their outposts, that meant military action was imminent.