Why do heavy quarks flow sideways in off-center collisions?

When copper and gold nuclei collide off-center at RHIC, the asymmetric collision geometry creates a tilted energy-density profile that pushes charm quarks sideways. This sideways flow (called $v_1$) is orders of magnitude stronger than in lighter particles and is exquisitely sensitive to both where charm quarks start and how strongly the hot medium drags them. Measuring this flow could pin down fundamental quark-matter properties.