The Swing Amplifier

Sketch of the orbital progression, right to left, of a leading arm-fragment that becomes a trailing arm-fragment. Notice that the rotation of the arm-fragment is retrograde, which is the same as the direction of the epicycles. Hence the epicycles are perturbed for a long period of time, and respond strongly.
Similar sketch to the one above, although here the orbital progression (which in reality is right to left) is shown left to right. Again, stars whose epicyclic phase place them in the feature are perturbed for a long time as the epicycle progresses, so does the arm.
Density contours of a leading arm unwinding to become a trailing arm. The swing amplifier boosts the arm strength considerably.