By his own admission from the USA Judo site, he coaxed his opponent into an illegal leg grab to win his match and put him in the semifinals:
"I was shaking my arm out and trying to figure out how I was going to beat this guy when I remembered from watching his other fight that he was always trying to grab [Takahiro Nakai's] leg when he stuck it out," Stevens said of his strategy to capitalize on a new rule that bans grabbing your opponent's leg during a match. "So I decided to go with that and I was able to trick him into grabbing my leg and trying to throw me with a te guruma [hand wheel throw] and it worked."
Stsiashenka's failed te guruma attack resulted in a hansokumake (disqualification), making him one of seven players to be removed from the tournament for violating the new rules.
I will re-iterate, that they need to add a shido for these rules (at least for the time being) to help set the tone. In this case, Stevens' opponent might have had the ability to change his game to prevent being disqualified (and if he didn't, he'd deserve it). At first, the IJF was reluctant to change the rules during the qualification period for the Olympics. If only one or two people had been dq'd by the new rules, I'd say that the effect was not as bad as I thought, but 7!