The collision of black holes at relativistic speeds probes gravity in its most extreme dynamical regime. We study head-on collisions of equal-mass black holes whose spins are antialigned with each other and perpendicular to the collision axis: the minimal configuration in which a net gravitational recoil is generated, purely spin driven and, by symmetry, one dimensional. Performing a sequence of full numerical simulations for dimensionless spin magnitudes $\chi =0.5$, 0.65, and 0.8 over a wide range of initial momenta, we follow the recoil velocity accumulated in time: it rises to a peak value $V_p$ at merger and then relaxes to a smaller final value $V_f$—an “antikick”, during which the direction of the emitted linear momentum reverses. In the relativistic regime, the peak and final recoils are found to be directly proportional, $V_p\approx 7.4V_f$, largely independently of both the initial momentum and the spin magnitude, pointing to a common postmerger relaxation. The antikick amplitude collapses onto a single master curve with an empirical spin scaling ${\chi }^{1.27\pm 0.08}$, intermediate between the linear dependence predicted by a zero-frequency-limit analysis and the cubic dependence of a close-limit estimate, both derived in the Appendixes together with a quasinormal-mode model of the antikick. These results complement the previously established maxima of recoil and radiated energy from grazing high-energy encounters [J. Healy and C. O. Lousto, Phys. Rev. Lett. 131, 071401 (2023)., J. Healy, A. Ciarfella, and C. O. Lousto, Phys. Rev. D 111, 084077 (2025).] and motivate sequences at higher spins and energies.

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• General relativity
• Gravitational waves
• Numerical relativity