Basically exactly what you saw. The sweeper lets a gap open at a critical time forcing others to burn a match or two just prior to the sprint.
I think Legion had this planned before day one thinking Cory for the overall due to him being more suited for the Riverside course. Meaning, I think day one Justin wanted that win then work more for Cory while still being up there to steal points. So for Justin or any other team mate, 2nd, 3rd, 4th was a secondary goal for the omnium. This move (intended or otherwise) accomplished that perfectly.