While searching labialization on the Wikipedia, it is easy to find these statements: 'Labialized sounds involve the lips <...> When vowels involve the lips, they are called rounded.'
In Russian, as in many other languages, there is a rounded vowel [u]. It is very typical of Russian people, who learn English, to pronounce English labialised (~rounded) [w] with the same state of the lips, look at the picture:
But when these Russian speakers come to a teacher who tells them not to use their lips at all and pronounce [w] in a 'smiling' manner, a question arises: if both sounds are labialised, why cannot one use his/her lips in the same way as in pronouncing [u]?
Well, what is the difference between the state of the lips while producing [u] and [w]? Why both sounds are 'labialised/rounded', but do not really resemble one another in terms of the lips' state? Is there a more useful recommendation on how to hear/see the difference between [w] and [u]?