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It is generally accepted that the fastest way to reorient magnetization is through precessional motion in an external magnetic field^,,,,,,. In ferromagnets, the application of a magnetic field instantaneously sets spins in motion and, in contrast to the inertial motion of massive bodies, the magnetization can climb over a potential barrier only during the action of a magnetic-field pulse. Here we demonstrate a fundamentally different scenario of spin switching in antiferromagnets, where the exchange interaction between the spins leads to an inertial behaviour. Although the spin orientation hardly changes during the action of an optically generated strong magnetic-field pulse of 100 fs duration, this pulse transfers sufficient momentum to the spin system to overcome the potential barrier and reorient into a new metastable state, long after the action of the stimulus. Such an inertia-based mechanism of spin switching should offer …