These 2 initials can be described on their own.
1. The Cantonese /w-/ & Putonghua [w] are more or less the same, as is the ‘w’ of English.
2. The Cantonese /j-/ is in fact Putonghua [y], which is identical to the ‘y’ in English.
The reason Jyutping chose this ‘j’ is to distinguish it from the letter combination /-yu-/ used to represent the rounded vowel. This would avoid a combination of ‘*yyu’ (transliterated as /jyu/ in Jyutping). The mechanism is, therefore, different from Hanyu Pinyin.
Since ‘j’ represents other sounds in both Putonghua & English, beginners must remind themselves how Cantonese /j-/ is actually pronounced.