Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities, such as mobilize (from the Tarkir: Dragonstorm release), are triggered abilities and will have "when," "whenever," or "at" in their reminder text. Replacement effects are unaffected by Twinflame Travelers's ability. For example, a 1/1 Elemental creature that enters under your control "with a +1/+1 counter on it" won't receive an additional +1/+1 counter.