  We study ultrarelativistic encounters of two spinning, equal-mass black holes
through simulations in full numerical relativity. Two initial data sequences
are studied in detail: one that leads to scattering and one that leads to a
grazing collision and merger. In all cases, the initial black hole spins lie in
the orbital plane, a configuration that leads to the so-called "superkicks". In
astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000
km/s; here, we find configurations that exceed ~15,000 km/s. We find that the
maximum recoil is to a good approximation proportional to the total amount of
energy radiated in gravitational waves, but largely independent of whether a
merger occurs or not. This shows that the mechanism predominantly responsible
for the superkick is not related to merger dynamics. Rather, a consistent
explanation is that the "bobbing" motion of the orbit causes an asymmetric
beaming of the radiation produced by the in-plane orbital motion of the binary,
and the net asymmetry is balanced by a recoil. We use our results to formulate
some conjectures on the ultimate kick achievable in any black hole encounter.
