  An "event-shape twist" technique is proposed to study the longitudinal
dynamics of harmonic flow, in particular the effects of rapidity fluctuation
and event-plane decorrelation. This technique can distinguish between two types
of rapidity decorrelation effects: a systematic rotation versus a random
fluctuation of flow angles along the rapidity direction. The technique is
demonstrated and the magnitude of the two decorrelation effects is predicted
using the AMPT model via a single particle analysis and two-particle
correlation analysis. An observed decorrelation can be attributed to a
systematic rotation of event-plane angle along the pseudorapidity, consistent
with a collective response to an initial state twist of the fireball proposed
by Bozek {\it et.al.}. This rotation is also observed for several higher-order
harmonics with the same sign and similar magnitudes.
