By touching the forehead of a target and then, according to a mental image the user forms, the target changes into a tree frog. This is mainly used for interrogating several prisoners of war. Seeing their comrades changed into a frog before their eyes, the prisoners of war will cave in, becoming what could be described as a "frog chorus", sonorously chanting information. This terrifying technique will, without a doubt, make an "information leak" open up.
The word for frog, "蛙", is pronounced "kaeru". The term "to turn into", written as "変える", is also pronounced "kaeru", which leads to this technique's interesting pronunciation, "Kaeru Kaeru no Jutsu".