  We use numerical relativity to study the merger and ringdown stages of
"superkick" binary black hole systems (those with equal mass and anti-parallel
spins). We find a universal way to describe the mass and current quadrupole
gravitational waves emitted by these systems during the merger and ringdown
stage: (i) The time evolutions of these waves are insensitive to the
progenitor's parameters (spins) after being normalized by their own peak
values. (ii) The peak values, which encode all the spin information of the
progenitor, can be consistently fitted to formulas inspired by post-Newtonian
theory. We find that the universal evolution of the mass quadrupole wave can be
accurately modeled by the so-called Backwards One-Body (BOB) model. However,
the BOB model, in its present form, leads to a lower waveform match and a
significant parameter-estimation bias for the current quadrupole wave. We also
decompose the ringdown signal into seven overtones, and study the dependence of
mode amplitudes on the progenitor's parameters. Such dependence is found to be
insensitive to the overtone index (up to a scaling factor). Finally, we use the
Fisher matrix technique to investigate how the ringdown waveform can be at
least as important for parameter estimation as the inspiral stage. Assuming the
Cosmic Explorer, we find the contribution of ringdown portion dominates as the
total mass exceeds ~ 250 solar mass. For massive BBH systems, the accuracy of
parameter measurement is improved by incorporating the information of ringdown
-- the ringdown sector gives rise to a different parameter correlation from
inspiral stage, hence the overall parameter correlation is reduced in the full
signal.
