Mouth Transition When Singing?

I heard a lot of people saying that we should sing vertically not vice-versa.. But i totally feel that it's a bit hard..As an example, EEeee vowel...it should be horizontal...not that wide though. Becoz i've tried to sing vowel Eee. in a vertical way but it automatically flips to falsetto ...what i said is towards the high pitch. Why is that so? Can someone explain what's the method. I'm a new male singer in a school band btw :-) Really need your help. Thanks in advance.,