  A stellar-mass black hole (BH) or a millisecond magnetar is believed to be
born as the central engine of Gamma-ray bursts (GRBs). The presence of plateaus
in the X-ray extended emission or afterglow of GRBs is widely accepted as an
indicator of magnetar central engine, particularly those with a sharp decay
(faster than $t^{-3}$), so-called internal plateau. However, an alternative
model, by taking the evolution of the magnetic flux at the BH horizon into
account, suggests that an internal plateau can also arise from a
Blandford-Znajek (BZ) mechanism powered jet (hereafter referred to as the BZ
jet). In this study, we propose that a precessional BZ jet would manifest a
Quasi-Periodic Oscillation (QPO) signature on the internal plateau and the
subsequent sharp decay. Such lightcurves cannot be readily explained by the
activity of a short-lived, supermassive magnetar, thus favoring a Kerr BH as
the central engine. The X-ray afterglow of GRB 050904, comprising nine flares,
is characterized by a QPO-modulated plateau and sharp decay, which can be well
reproduced by a precessional BZ jet model. Therefore, one potential clue for
distinguishing between these two engines lies in whether QPO signature
throughout the entire plateau and the subsequent sharp decay, as the magnetar
scenario suggests a collapse at the end of the plateau.
