In spoken English, the movement of the tongue between /uo͞oue/, /ʊo͝ooo/, or /ɔôaw/ and other vowels can incidentally create a [w] sound. This extra sound occurs in transition between the vowels. For example, the word hour can be pronounced with the diphthong /ɑʊowow/ and may be followed by the vowel /ɚûrur/. New hearing cuers may notice a [w] sound occuring between the two vowels. This transitional sound is referred to as instrusive [w]. It is, however, not a consonant phoneme. It is simply a byproduct caused the the sequence of vowels. One would not cue the word hour using handshape 6.
Instrusive [w] is generally shown with brackets which indicate that the sound is phonetic. This is different than the phoneme /w/, which is written between slashes.
Understanding the difference between phonetic [w] and phonemic /www/ can be abstract and difficult to grasp. An easy approach for instructors to help students discriminate the difference is to ask them simply to take it out and see if the word changes. If one attempts to remove the [w] sound from hour, flower, power, sour, or Jewish, they will find that the sound comes back on its own. The [w] cannot be taken out of these words without inserting an unnatural pause between the two vowels that surround (and create) the transition. However, consider the /www/ in words like quit, quite, and quick. For these words the /www/ is a phoneme and an essential part of the word that must be cued. Removing the /www/ in these words will change them to kit, kite, and kick. This change helps cuers to see that the /www/ phoneme in a word like quit is fundamentally different from the [w] sound in the spoken word sour.