You don't use it manually, it's a cheap cleansing alternative used to temporarily cleanse characters during story quests.
For example, let's say you TF'ed Artoria but haven't finished the story yet. When you try to start a scene, the game will automatically prompt you to use the "free trial" to temporarily cleanse her. After the scene ends, you'll still have your TFed Artoria.
Basically, it's a cheap quality-of-life item, so you don't have to fully cleanse and recorrupt characters yourself every time.