  We present the first fully-nonlinear numerical study of the dynamics of
highly spinning black-hole binaries. We evolve binaries from quasicircular
orbits (as inferred from Post-Newtonian theory), and find that the last stages
of the orbital motion of black-hole binaries are profoundly affected by their
individual spins. In order to cleanly display its effects, we consider two
equal mass holes with individual spin parameters S/m^2=0.757, both aligned and
anti-aligned with the orbital angular momentum (and compare with the spinless
case), and with an initial orbital period of 125M. We find that the aligned
case completes three orbits and merges significantly after the anti-aligned
case, which completes less than one orbit. The total energy radiated for the
former case is ~7% while for the latter it is only ~2%. The final Kerr hole
remnants have rotation parameters a/M=0.89 and a/M=0.44 respectively, showing
the unlikeliness of creating a maximally rotating black hole out of the merger
of two spinning holes.
