Fukamushicha is deep-steamed sencha--asamushi = light-steamed, chumushi = mid, and so forth. The more steam used the greater the flavor and the smaller the leaves; fukamushi can look a lot like tea dust.
Personally, I find fukamushi to be the hardest tea to brew correctly, and I am still working on it. But the few times I got it just right, it was amazing, though more often than not I'll screw up at least one infusion from any given session.
Today I experimented with lower temperatures, hoping to create a sweeter tea. It worked the first time, but later I let the water get too cool (140-ish) and the tea became too weak.
The dry aroma is thick and rich and vegetal.
The much smaller leaves of fukamushi can prove to be too much for a sasame filter. My teapot gets rather clogged about a third of the time.
Taste. Honestly, I just don' like this one. There are many who disagree with me, but I think the Fukamushi Maki from Den's was much better. I have used about 90 grams so far, and I have yet to produce a cup of this stuff that I liked. Its strong and flavorful to be sure, but lacks the sweetness, complexity, and sass that I look for in a good fukamushicha.