Fukamushi is a type of sencha that is subjected to longer/deeper steaming during the processing of the tea leaves. This results in smaller pieces of tea in the final product, which in turn results in a sweeter and more "full bodied" taste relative to regular sencha. You can see some of the fine particles of tea dust in the following picture.
The back of the package includes Hibiki-An's preferred brewing recipe. You can customize the flavor of the tea by varying the amount of tea and water, the water temperature, and the brewing time. I don't have a special fukamushi brewing vessel; I use my standard kyusu, which probably isn't ideal because the strainer doesn't catch all of the tea particles during the pour.
I won't try to describe the flavor profile in detail, because the taste can vary from day to day depending on how I brew the tea. Generally speaking, I would say that the tea is balanced with a slightly sweet aftertaste. Not bitter, and not too "grassy" or "veggie" tasting. In view of its relatively high price, however, I'd hesitate to buy this tea again. That said, I would gladly accept a free bag or lifetime supply from Hibiki-An if they decide to sponsor me.