A spool of constant rotational inertia \(I_0\) is mounted on a fixed horizontal axle with negligible friction. A string wrapped around the spool is pulled such that the spool rotates from rest in the positive direction. A sensor records the spool's angular velocity \(\omega\) as a function of time \(t\), producing the graph shown from \(t = 0\) to \(t = t_1\). Which of the following statements correctly relates a feature of the graph to the physical behavior of the spool during this time interval?