Most of Trover's Active skills tell you what to do with them when used. The majority of them go to the Heatup Track when used. However, four Active skills have no such instruction: Just Sit Back, Natural Resources, Thickest Skin, and Through the Flames.
For these skills, the intention is that the skill simply stays in the Active slot. Its effect can be triggered multiple times, until the end of the battle or until you voluntarily remove it from the Active slot.