When soccer superstar Cristiano Ronaldo attacks defenders on the playing field, he does so with a quickness that leaves their heads spinning. Before they know it, they've lost.

A controversial investment brokerage Ronaldo has been advertising on Twitter can sometimes have a similar effect on investors, according to a report from Donal Griffin of Bloomberg News.

Offering products called "contracts for difference" (CFDs), Exness Group allows investors to make highly leveraged trades that can provide outsized gains on the way up, but also result in deep, swift losses in the event of a downturn.

The firm, based in Cyprus, allows clients to borrow funds totaling up to 500 times their original deposits. Critics of the practice say investors often don't understand the potential downside associated with such bets, leaving them vulnerable to huge losses.

Happy with my new partnership with @EXNESS https://t.co/TjFu0u27UI — Cristiano Ronaldo (@Cristiano) August 2, 2017

For an example of how such a trade can work, imagine you wager $100 on a bullish S&P 500 exchange-traded fund contract. If you take a CFD up on the 500-times leverage it offers, the notional value of your investment increases to $50,000. Then, if the ETF gains 0.5%, you make $250. But if it declines 0.5%, you lose the same amount, and already find yourself owing $150.

To provide an idea of dismal rate of success, a study cited by Bloomberg shows that CFD users in Spain lose money 82% of the time.

But Ronaldo is not alone in his endorsement of Exness. Real Madrid, the club team for which he plays, has also partnered with the firm. In addition, Exness has been a sponsor of Red Bull Racing for multiple years.

According to Griffin's report, CFDs were called "a volatile form of gambling" by an Irish judge in 2014. Retail investors in the US are mostly barred from using them, but in Europe they represent the final frontier of highly risky, lightly overseen financial speculation, Bloomberg says. And that's drawn the ire of regulators.

Retail traders in the US may be largely blocked from using CFDs, but there are other ways for them to place highly leveraged wagers. There's an entire sub-industry of leveraged ETFs available for them, with the first funds offering quadruple-sized stock returns approved by the US Securities and Exchange Commission earlier this year.

In another area of the market, investors have increasingly been using leveraged products to bet on volatility. Betting against price swings has even become one of the market's most crowded trades. It's a trend that has some market experts worried — even going as far as to compare the current situation to conditions leading up to the 1987 stock market crash.

While Ronaldo has yet to hawk these types of leveraged product, don't rule it out yet. After all, he raked in $35 million from endorsements last year — and has 59.6 million Twitter followers.

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