Fresh Hop beers are an amazing thing. Instead of utilizing pelletized hops, the brewer uses fresh-off-the-vine full cone hops that go directly into the beer. In the case of Sierra Nevada Southern Hemisphere, the hops are taken directly from New Zealand and flown to the brewery in California for immediate utilization. Using hops from New Zealand is a unique twist on the style as the base beer would be considered an American IPA.
Here's an excerpt from Sierra Nevada's website about the beer:
Our newest addition to our Harvest family is Southern Hemisphere Harvest. This is the first time we know of that an American brewer has put out a beer with fresh-picked hops from the southern hemisphere. The inaugural ale will debut in late April and will feature fresh Pacific Hallertau, New Zealand Motueka and New Zealand Southern Cross hops, all from New Zealand.
Like our Celebration Ale, the fresh hops in this beer are dried right after being picked then shipped immediately to Chico for brewing so that they retain their peak aromatics and flavors. To ensure the freshest hops possible, we went to the added expense of flying these hops from New Zealand to Chico so we could brew with them the week after they were picked.
If you've never had a Fresh Hop beer, you should seek one out immediately. Southern Hemisphere should be pretty widely available, but other great examples include Founders Harvest Ale and Great Divide Fresh Hop Pale Ale.