N. V. Tatsenko introduction to theoretical phonetics of english

Approximants or glides include bilabial [w] and palatal [j]: [w], as in why, starts out with the lips firmly rounded, these articulators then moving away (= gliding) from the narrowing in the mouth. When articulating [j], as in you, the front part of the tongue is first raised towards the hard palate, then the soft
palate is raised and the air goes along the central part of the tongue. The vocal cords are kept together and are vibrating.
The most important thing to remember about these phonemes is that they are phonetically like vowels but phonologically like consonants (in earlier works on phonology they were known as "semivowels"). From the phonetic point of view, the articulation of [j] is practically the same as that of a front close vowel such as [i], but is very short. In the same way [w] is closely similar to [u]. If you make the initial sound of 'yet' or 'wet' very long, you will be able to hear this.
The reason why these sounds are called semi-vowels is thus their manner of articulation: like true vowels, semi-vowels are produced without a major obstruction, i.e., there is a wide gap between the active and the passive articulator, so that the airflow can escape relatively freely from the mouth. However, unlike true vowels, semi-vowels never form the nucleus of a syllable (e. g., week, yellow) and are therefore usually considered consonants.
Besides, they only occur before vowel phonemes; this is a typically consonantal distribution. We can show that a word beginning with [w] or [j] is treated as beginning with a consonant in the following way: the indefinite article is ‘a’ before a consonant (as in 'a cat', 'a dog'), and 'an' before a vowel (as in 'an apple', 'an orange'). If a word beginning with [w] or [j] is preceded by the indefinite article, it is the 'a' form that is found (as in 'a way', 'ayear'). Another example is that of the definite article. Here the rule is that 'the' is pronounced as [бэ] before consonants (as in 'the dog' [бэ dug], 'the cat' [бэ kst]) and as [6i] before vowels (as in 'the apple' [6i spl'], 'the orange’ [6i urind3]). This evidence illustrates why it is said that [j], [w] are phonologically consonants. However, it is important to remember that to pronounce them as fricatives (as many foreign learners do), or as affricates, is a mispronunciation. Only in special contexts do we hear friction
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noise in [j] or [w]; this is when they are preceded by [p], [t], [k] at the beginning of a syllable, as in these words:

'pure'