Yin is more taste. Yin is descending energy. Yin is more cool. Yin is inactivity.
Yang is more smell. Yang is ascending energy. Yang is more warm. Yang is activity. Because yang is active, it is related with qi.
The longer (and usually slower) tea leaves grow on the bush the more warmth, and therefore yang they acquire. This says much about the warmth generated from certain flushes and seasons. Also, it is common knowledge that the older the tea tree, the more yang it generates.
The process of oxidization also generates heat. Oxidization occurs naturally and at a slow (longer) pace in the aging of tea, this creates heat, yang, qi.
The natural process of aging teas such as puerh generates wonderful smells and odours. These odours are yang, heat, and an indication of chaqi.
Conversely, the chaqi of teas that don’t age well such as white, green, and red tea, is the strongest when they are the freshest. This also happens to be when they smell the most fragrant, contain the most yang and purest chaqi. As these teas age and oxidize they loose their fresh scent and their qi- the energy of these teas can no longer rise as strongly.
Tea that ages well such as puerh also goes through an initial phase of loosing its wonderful smell and qi.
For fresh puerh, the qi and odours are strong for the first few years after production but then seem to wave before becoming stronger again years later. Years later as puerh ages, it generates much heat, yang, qi, though the micro bacterial processes of aging. Because the changes that occur during aging takes place very slowly- it generates lots of qi. As chaqi is generated once again, its odour becomes quite strong.
This is how “smell” can be used as an indication of chaqi.