Does Strionic Resonator copy the trigger or the effect of a triggered ability?

Strionic Resonator's activated ability targets a triggered ability on the stack and copies it. This means it copies the entire triggered ability as it exists on the stack, including any choices already made for it (such as targets chosen when the trigger was put on the stack). The copy is itself a triggered ability on the stack and resolves separately. (CR 706.10 governs copying objects on the stack.)

The distinction matters: you are not merely duplicating the result of the ability; you are creating a second instance of the ability that will resolve independently. Each copy can have its own targets chosen at the time the copy is created (CR 706.10a allows you to choose new targets for the copy).

The triggered ability must already be on the stack when Strionic Resonator's ability is activated — you can't preemptively copy a trigger before it has triggered. The Resonator's ability is itself an activated ability, not a triggered one, so timing rules for activated abilities apply (CR 402.2 isn't relevant; see CR 602.1 for activating abilities).

Example: Your Mulldrifter enters the battlefield and its "draw two cards" triggered ability goes on the stack. You activate Strionic Resonator targeting that trigger. A copy of the trigger is placed on the stack. When both resolve, you draw two cards twice — four cards total — because each instance of the triggered ability resolves separately.

Note that Strionic Resonator cannot copy activated abilities or static abilities — only triggered abilities (those that begin with "when," "whenever," or "at"). Attempting to use it on anything else is not a legal target (CR 603.1).