Ratio of ram pressures upstream and downstream to the shock for a galaxy moving in the same direction, but faster than the shock. For the galaxy velocity smaller than ∼1.5 times the shock velocity, the ram pressure increases when the galaxy takes over the shock (here, the upstream gas is assumed to be at rest). For a radio-galaxy with a bent tail, this means that the tail becomes narrower. For higher velocities, the tail becomes wider instead. The three curves represent M = 1.5, 2, 3 (black, red, and blue, respectively).