Some spells and abilities say that you “may pay” a certain amount of {E}, then describes an effect that happens “if you do.” In that case, no player may take actions to try to stop the ability’s effect after you make your choice. If the payment is instead followed by the phrase “when you do” followed by an effect, then after you choose to pay {E}, a new triggered ability called a reflexive trigger is put on the stack, and you choose targets for the reflexive trigger (if the effect asks for targets). Players may respond as normal to this new triggered ability. Once that reflexive trigger resolves, the effect will finally occur.