This once relatively obscure category was associated with Anhui and Hunan, specifically with two rare and pricey teas from those provinces. Historically, the color yellow was reserved for the imperial court (viz. emperor’s robes, etc.) and so even today, the color carries not a little cachet in gifting and in the promotion and marketing of this group of teas.
The temperature surrounding the leaves, of course, varies as the leaves rest before the final drying. Due to the covering, there is also less evaporation or dissipation of the components in the leaves that yield aroma. The moisture and heat act on the polyphenols and chlorophyll in the leaves, and as the chlorophyll breaks down, the leaves take on a yellowish hue (just as with green leafy vegetables if kept too long).
This added step of covering small piles of leaves allows the warm temperature and moisture to act upon the leaves, eliciting a more complex flavor than would be the case for standard Green tea processing. Rolling and drying follow.
Courtesy of Lydia Kung