This page explains how to use Trap Elements in Chrono Cross: The Radical Dreamers Edition.
Trap Elements (identified by a “down arrow” icon in their name) are a type of Element that can be placed on the battlefield to capture a specific Element, as specified by the name of the trap. Eg. an Inferno Trap is used to trap the Inferno Element.
How Trap Elements Work
Like regular Elements, you need to set a Trap Element on the Element Grid. Unlike most Elements, Trap Elements have a singular fixed Level, starting from Level 5. So they cannot be placed in higher or lower Level positions.
In battle, after building your Elemental Power Level to the required amount, select the Trap Element to use it. Once used, it’ll affect all foes on the battlefield–and a Trap indicator will appear near the top-left corner, by the Field Effect display.
From this point, whenever a foe uses the Element that the trap works on, that Element will automatically fail (dealing no damage) and be added to your inventory. After the battle, you can then assign this Element to your characters like usual.
If you know or can predict what Elements a foe will use, Trap Elements are a great way to mitigate damage, while providing you with powerful (and often hard-to-obtain) Elements!
How to Obtain Trap Elements
You can find some Trap Elements from chests, but the easiest way to stockpile them is to buy them. Over at Marbule, the demi-human village situated towards the south-east, there’s a guy directly ahead of the entrance who sells Element Traps.