This exercise demonstrates a ‘back vowel’ action. The tongue is position away from the teeth, and the jaw is extended. The ‘Yo Yo’ exercise is to demonstrate the type of acoustical sound the mouth offers. ‘YoYo’ is generally a duller sound in comparison to ‘ee’. When used in contrast, it can be a compelling sound when gaining the attention of an audience. Practice well and will feel the difference in singing words.