After harvest green tea leaves need to be de-enzymed to prevent them from oxidizing which would result eventually in them becoming a version of black tea. In Japan de-enzyming is done by steaming the leaves after harvesting.
Most sencha ("steamed tea") is steamed for around a minute to adequately prevent oxidation.
Tea from the Makinohara Plateau can be steamed almost twice as long as Sencha usually is without the fiber breaking down the way most teas would. The result of the deep-steaming process is an extra savory-soupy tea broth with a satisfying character that is almost addictive. A fabulous tea you just don't want to stop drinking.