The move is known as an "oil check," and soccer isn't the only sport where it happens.
Rodrigo Baldasso da Costa, a center back for Ponte Preta, twice stuck his fingers into the rear end of Vittoria player Rodrigo Tellez, according to News.com.au. A referee reportedly issued the offending player a red card, writing in his match report: “I sent off ... Rodrigo ... after I was informed by the fourth official that the athlete had introduced his middle finger twice between the buttocks of his adversary number 22, Santiago Trellez.”
If you follow soccer, you may know that this has happened before. During the 2015 Copa America quarterfinals, Chile's Gonzalo Jara infamously put his finger in the butt of Uruguay's Edinson Cavani, prompting an investigation into the incident. Jara was ultimately slapped with a three-game ban.
The move—colloquially known as the "oil check"—routinely crops up in other sports, too. It's most common in Brazilian Jui-Juitsu, where it's not technically against the rules, and it's used to gain an advantage over an opponent by forcing them to change their position to protect their ass (literally).
On an episode of his podcast in 2014, Joe Rogan discussed the use of the oil check in MMA and wrestling. "Sometimes in the middle of grappling a guy will literally have his fingers deep in a guy's asshole, just digging in there, and it forces the guy to move," he said. "They've got to move in a way to get away from that grip."
The sport of wrestling has had controversies involving athletes putting their fingers in other people's butts. Back in 2010, a California wrestler was charged with sexual battery after doing it to one of his teammates.
"It is never acceptable to insert fingers into the opponent's anus (regardless of duration)," the National Wrestling Coaches Association said at the time, according to Deadspin.
In soccer, the move is definitely not allowed. Baldasso da Costa was promptly ejected from the match with a red card. At the time of the incident, Ponte Preta was up 2-0, but Rodrigo's insertion (and then ejection) apparently cost them dearly. Vittoria won 3-2—prompting hordes of Ponte fans to storm the field.
Needless to say, it was an eventful match.