Some monsters can never truly be tamed by a rider and, during the heat of battle, can be prone to giving in to their wild and frenzied nature.
During the Start of Turn sub-phase of each of their turns, this model must make a Leadership test (using its own unmodified Leadership). If this test is passed, the rider is able to keep control of their mount. If, however, this test is failed, the rider has lost control and their mount becomes subject to the Frenzy special rule until their next Start of Turn sub-phase.
Note that the rider does not gain a +1 modifier to their Attacks characteristic.