They looked at 4 steps that were taken in Wuhan (school closures, closing non-essential services, shelter-in-place, and major travel restrictions) and used that as an assumption in their predictions. They actually only assumed that 3 were necessary to achieve the same decreases as Wuhan. Anyway, it's basically a mathematical model based upon conditions when these steps were taken. New York was further along before they shut down, so their peak is higher.