Reflexive and delayed abilities, and answering a play trigger

Some abilities do their work in two stages. A reflexive trigger is a second ability that a spell or ability creates while it resolves, and it goes on the chain at once as a separate item. A delayed ability waits for a window of time that the effect names, such as the end of the turn or the next time something happens. Both are easy to mistake for part of the original effect, and the mistake costs you a chance to respond.

A reflexive trigger is recognized by the phrase "Do this:" or "Do one of the following:", usually after a condition (rule 388, rule 388.1 and rule 388.2). When the condition is met a new ability is created and added to the chain as a Pending Item (rule 388 and rule 389.1). "Do this" can be followed by "N times", which adds the trigger N times, and "Do one of the following" names which of the related triggers goes on the chain (rule 388.1.a and rule 388.1.b). If it begins with "you may", its controller decides when it is finalized, and declining removes it from the chain (rule 388.4). The key consequence is about choices: you do not choose targets for a reflexive trigger while you play the card that creates it. You choose them when the trigger itself is put on the chain (rule 355.5.b). Take Dragon's Rage, whose current text reads "Move an enemy unit. Then do this: Choose another enemy unit at its destination." When you play it you choose only the unit to move and where it goes. If it resolves, the second unit is chosen when the reflexive trigger goes on the chain, where it can be answered separately.

Delayed abilities can be any other kind of ability, with the added property of a window in which they are active (rule 391 and rule 391.1). A delayed trigger names a time of the turn or a frame of time, a delayed replacement is worded "the Nth time" or names a time, and a delayed passive ability applies only for a stated window (rule 391.2, rule 391.3 and rule 391.4). They resolve or apply just like the ability they augment, but only in that window (rule 392). They are created by other abilities and not tied to units or gear, so they happen when their time comes even if the source has left the board (rule 393). Imperial Decree reads "When any unit takes damage this turn, kill it." That is a delayed trigger whose window is the whole turn, so it can kill several units.

Now a play trigger. When you play a unit or gear, it is finalized and then resolves immediately, so nobody can respond to the card itself (rule 337.2). Its "When you play me" ability is a Play Effect that goes on the chain only after the permanent has entered the board (rule 383.4.a.2). Once it is finalized, the controller of the next item on the chain gains Priority and players can play legally timed Reactions in answer (rule 337.4 and rule 338.1.a). The trigger is a separate object from its source (rule 355.9.c), so removing the unit does not remove the trigger. What changes is the information the trigger looks at. A referent such as "my Might" or "here" is checked when the instruction executes (rule 359.3.f.2), and a unit that has left the board has null Might and no location (rule 359.3.e.12). So if Carnivorous Snapvine's controller plays it and the opponent kills the Snapvine in response to its trigger, "we deal damage equal to our Mights" has nothing to read and is ignored. If Harnessed Dragon is killed in response to "kill an enemy unit", the trigger still kills one, because that instruction never looks at the Dragon.