Sengu is basically a relocation for a deity.
Whether it be praying, or making daily offerings, the Japanese have many ways of paying respect and homage to their Kami-sama, and amongst them, the most carefully executed and grand event of doing so, is the Sengu. The people build a new home (shinden), and relocated the deity that had lived in the previous one. Building the exact same building on the land right next to the old one is the common way, but you will sometimes see a stronger structure or new style of decoration on the new one as well. It is not something that can be done often, so it usually only happens once in a span of many years. That one time is called Shikinen Sengu.
Insight on Shintoism from Florian Wiltschko
Born in 1987, in Linz, Austria.
From a very young age, Wiltschko took an interest in Japan and gained knowledge through reading. When he was 14, he came to Japan on a family vacation for the first time, and visited many Shrines and Temples, which raised his level of interest even more. In 2007, he entered the Ueno Tenmangu Shrine in Nagoya, and studied about Shinto priesthood while living there. Upon returning home, and graduating from Vienna University in Japanese Studies, he moved to Japan for good and entered Kokugakuin University and majored in Shinto Studies. After graduating from his special training in 2012, he was appointed "gon-negi" (lowest ranking priest) at the Konno Hachimangu Shrine in Shibuya, Tokyo, fulfilling the position for 4 years. From May of 2016, he will continue his priesthood at Nobeno Shrine in Tsu City, Mie Prefecture.