It makes comments at specific times. For example, when you steal something, sometimes when you break an object, or the first time you equip a two-handed weapon. Basically, any scenario where the trunks can rag you about something - it's like your own private heckler.
Keep in mind that for the first few days following launch the trunks did not trigger any voiceover audio (now they do), making it difficult to see its comments as the text would just appear over your character for a second or two.
