Brewing tea and controlling variables to get the caffeine and flavors that you want.
"When you steep tea, "the caffeine comes out first" from the leaves, says chemist Kuhnert. If you keep infusing, compounds called thearubigins seep out of the plant, and some will actually bind to the caffeine, he says — meaning the caffeine can't then bind to your brain receptors and wake you up. The longer you infuse the tea, the less caffeine that's available to your body, so a shorter brew can also be more invigorating, says Kuhnert."
While this article discusses English tea making, it does explain a few things about Chinese tea rituals (such as the use of really fine, delicate cups, fussiness on water temperature, and the chucking out of the first brew. It's the second pour that is desired, because it has the right flavors and less of the undesirable molecules, like caffeine).