It's sort of like percussion, but you wait for the right moment to play it.

So... say we're doing 'Are You Feelin' Alright?' When we get to the end of the song, it goes...

You ain't feelin' alright, no. Come on, are you feelin' alright? [ She makes a gesture signifying that she's hold the triangle by a string, then uses her other hand to hold the invisible stick and pretends to tap it against the also invisible triangle. ] Ding!