VOICED PALATAL UNROUNDED SEMI-VOWEL [j]
Tiberian Hebrew yodh is pronounced like English "y".
GEMINATED VOICED PALATAL STOP [ɟɟ]
Yodh with dagesh (i.e., doubled/geminated yodh) is not just held for a longer duration than yodh without dagesh, but it is also pronounced stronger. When you pronounce regular yodh, you let the air continue to flow just like when pronouncing English "y". However, when you pronounced yodh with dagesh, you need to stop the airflow almost as if you were pronouncing "g". This is actually a very important pronunciation difference to observe, since it occurs so frequently in the 3M vayyiqtol verbal forms.
SOURCES AND FURTHER READING
Khan, Geoffrey. 2020. The Tiberian Pronunciation Tradition of Biblical Hebrew. Cambridge: Cambridge and Open Book Publishers. §I.1.10.