Footage taken from the Mr. Olympia 2015 backstage where the mass monsters pump themselves up prior to heading on stage.
The main purpose of pumping up backstage before a contest is to create blood flow into your muscles allowing a more vascular and tighter look onstage. Throughout the day leading up to the contest, the contestants would be eating a higher amount of carbohydrates with their meals filling their muscles out. Their glycogen levels would have been very low due to the hardcore dieting undertaken during their contest preparation, allowing them to respond better to higher carbohydrate and sugar intakes. They have this down to a science!