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According to Wiedenhof's A Grammar of Mandarin, page 42,
The pronunciation of the final -ui [weı] vary with the accompanying tone for some speakers. With first or second tone, the u sound tends to be a vowel, followed by a schwa and a semivowel: -ui [uəj], as in tuī ‘push’ and huí ‘return’.
How is final [j] pronounced, especially after a vowel?
There may be a difference between what an author writes as "ai" versus "aj", but this is usually a substitute for the difference between a sequence of vowels in different syllables versus a vowel plus high front vocoid in the same syllable. The difference could be written as [ai] versus [a.i], but writing syllable boundaries is not a popular option especially in a practical orthography.
There actually is a phonetic difference in North Saami between [u:C] (=[uuC]) and [uwC], which is auditorily very elusive, which is realized in subtle low-level differences in fricative noise in the case of [w] before certain consonants. In your case, the author seems to be saying something about low-level phonetics, and really there is no substitute for listening to examples – which isn't possible in this case. I would expect the duration of the [uj]-transcribed token to have a shorter duration on the palatal segment, compared to a token transcribed with [ui]. Generally, [j] has a much shorter duration compared to [i], also the constriction is often greater. He makes a similar statement about iu and tone two pages before – perhaps there is some phonetic study of the difference in Mandarin. I would expect the difference to be about duration.
[j]in "fly", "may", and "boy".iandjis generally a phonemic one, not a phonetic one, because nobody can really agree on a universal phonetic definition of "syllabic" or "vocalic" (especially in diphthong contexts).