In addition to caffeine, tea also contains:
, which is reported to have pyschoacive properties.
, which is a compound similar in structure to caffeine.
, which is also a compound similar to caffeine.
Although present in small amounts, I can't help but think these compounds have something to do with the way tea can make one feel. It's also quite reasonable that these compounds could be present in varying amounts in different tea plants, which could help explain the variation in Qi from one tea to another.
I know I am a bit late in the game, but I want to throw in my 2 cents! I have been studying martial arts, Taoism, Buddhism, and tea in Korea for a while now, and qi has played a significant role in my educations.
I think that the quote up here is dead on, but also a bit hollow. We can take the reductionist approach so common when Westerners try to understand these kinds of ideas, and in many ways I think the reductionist approach is great, but I think that it misses the deeper, more spiritual aspects. People from cultures all over the world have spiritual experiences, sometimes from substances like tea, food, or even peyote, and sometimes from meditation, church, or music, and sometimes spontaneously. What is art if not a manifestation of what words cannot capture? I think the concept of qi is trying to explain to us how we work, giving us a tool through which to understand the world. (not to mention some cool Japanese cartoons)
So for chaqi, though I have read some skeptics and haters on teachat referring to it like the Emperor's Clothes, I have to vote in on the "very real" side of things. When tasting or buying tea, I look for how it makes me feel above how it tastes. My teachers here always ask me first how the tea makes me feel, then other questions. When Mike Harney says tea should make you smile, I interpret this to be what he means.
Also, someone noted feeling relaxed and alert, rather than just amped up. I was taught that the caffeine in puerh changes during fermentation, and so it metabolizes differently, causing different sensations. I don't drink coffee anymore because it makes me anxious. Tea, especially puerh, does not! But it does make ㅡe feel stronger, more alert, sharper, optimistic, and, most importantly, it fills up my energy reserves that get drained during heavy training. I consider these energy reserves to be qi (in Korea we talk about the "danjeon", the fountain of qi at the bottom of your core), and a good, aged sheng fills up my qi reserves like nothing else! I drank a 1960 loose leaf sheng that made my danjeon feel like it was on fire! Though I have noticed that all teas have some level of qi, the older puerhs ones have more, and shengs more than shous.
Last thought: learning about the directional flows of chaqi, I was taught that puerh pushes qi down, which is why a lot of people feel tired from it. I was also taught to drink oolong before puerh (opens up your "shinpo", a meridian line or some such along your chest), and maybe green tea after (pushed your qi in a circular motion), but not in reverse order. When I have tried puerh before oolong, it makes me feel a bit weaker and a bit sick. Interesting!