Rule 40 – which PRs may be familiar with from previous Games – means athletes and brands can get in trouble for using words such as "victory", "medal", "challenge", "performance", "effort" and "2016", depending on the context, from 27 July until midnight on 24 August.
This is done to prevent so-called ambush marketing and originally came about "to preserve the unique nature of the Olympic Games by preventing over-commercialisation", as well as protecting the official sponsors, which shell out millions for exclusive marketing rights.
It also means that firms such as kit suppliers may not be allowed to retweet athletes' posts, or pass on messages of good luck for the Games.
For Rio, the International Olympic Committee has relaxed the guidelines somewhat to allow "non-Olympic advertising" and athletes were permitted to post on social media about their non-official sponsors, as long as they steered clear of using any Olympic properties or references.
Applications to advertise during the blackout period have to be submitted months before Olympic teams are chosen and for Rio, there was a requirement to begin running the approved ads by 27 March 2016.
However, the rules are still complex, and the relaxations will will be monitored closely by the official sponsors for any possible infractions.
Under Armour, which has American swimmer Michael Phelps and Andy Murray on its books, among others, is not one of the official sponsors, but has various activity renting a series of outdoor gyms on a 50-mile stretch of beach in Rio to set up marketing outposts and allow fans to take part in daily workouts.
John Lewicki, who oversees global Olympic sponsorship deals for McDonald’s, told Reuters he would be using these Games to assess the value of future deals. "I wouldn’t necessarily say we’re all happy about it," he said. "If we find Rule 40 impacts the value of our sponsorship, we could always go back and renegotiate for the future."
It will give the official brands cause for concern, particularly if agencies continue to get more creative with how they use their big-name clients. A few years back, Good Relations managed to cook up an alternative way for Subway to use 2012 Olympic silver medallist Louis Smith. The gymnast, along with several other athletes including boxer Anthony Ogogo, rugby player Tommy Bowe and pole-vaulter Holly Bleasdale, took part in a campaign describing what made up their favourite sandwich – their 'Personal Best'.
While Rule 40 is not new, it has been highlighted as the Olympic movement deals with the fallout from a ban on Russian athletes over doping allegations. Former British Olympic heptathlete Kelly Sotherton suggested the IOC was more concerned with punishing athletes over sponsorship infringements than doping.
And while many athletes are not subtle about plugging their sponsors, some were more vocal than usual ahead of the enforced blackout.
Some were more light-hearted about the law than others...
First rule of the Olympics: you don't talk about the Olympics. #Rule40— Brent Lakatos (@BrentLak) July 27, 2016
While Kris Mychasiw, sports agent at Sprint Management, was a little sceptical about whether the IOC would actually take action on those infringing the rule.
Rule 40 should be abolished. Let athletes make a living & be proud to provide value to sponsors & partners. #Rule40— Kris Mychasiw (@Mychasiw) July 27, 2016
For those who have paid up for a chance to be an official sponsor, plenty of time and preparation has gone into the campaigns. PRWeek spoke to big brands such as Coca-Cola and P&G to hear what they have lined up.