Changing phases (ex: from 2nd main phase to end phase) is a consensual game action, meaning each player MUST agree to the change in phases. Therefore, if I have mana available, instants in my hand, and no other cards/effects on the field, it is PREPOSTEROUS that my opponent was able to skip directly from main phase 1 to end step and trigger his commander's effect WITHOUT giving me any priority window whatsoever... I should have had a MINIMUM of 3 priority windows, during which I could have casted Rapid Hybridization