In the widely viewed confrontation between a swarm of motorcyclists and a family in a Range Rover on the Henry Hudson Parkway, Christopher Cruz played a minor but key role.

He appeared early in videos of the 2013 episode, and did not take part in the violent climax, when several bikers were seen pummeling the driver after catching up with him on West 178th Street.

Nonetheless, Mr. Cruz, 29, pleaded guilty to second-degree assault and riot in State Supreme Court in Manhattan on Wednesday in a deal with prosecutors. In return, he was promised a sentence of four months in jail and five years’ probation, far less than the two and one-third to seven years in state prison he faced had he been convicted by a jury.

It was Mr. Cruz, prosecutors said, who slowed his motorcycle in front of the Range Rover on Sept. 29, 2013, and forced it to stop, so other bikers could surround it. A videotape shows Mr. Cruz looking backward as the car’s fender bumped his back wheel. He then dismounts and approaches the car.