Well, here are babelcarp's returned definitions:
yun = (云 or 雲)  cloud(s), or short for Yunnan; or (韵 or 韻)  literally Rhyme, but in a tea context, Aftertaste
hui gan = (1) pleasant aftertaste, literally Returning Sweet (回甘) [2,1]; or, much less commonly, a less specific label for virtually any effect after the liquor has been swallowed, literally Returning Feeling (回感) [2,3]
So, perhaps colloquially they are often used for "aftertaste" but there are different shades of meaning to the term.
BTW I like Cary's Christmas cap