Sage Transformation (仙人化, Senninka) allows the user to use natural energy to gain increased physical abilities and perform senjutsu. Sage Transformation allows its users to wield natural energy without the risk of being turned into a statue, as those who failed to learn Sage Mode do.
Jūgo's Clan Variant
Jūgo's Clan's Sage Transformation is the result of an unnamed ability which allows them to passively absorb passively absorb natural energy, resulting in increased physical abilities and the possibility to perform various shape-shifting feats. This transformation is a multifaceted ability that permits them to achieve feats ranging from creating various weapon-like appendages to even consuming other human beings, as well as a general increase in their physical capabilities. In the experiments created by Orochimaru, this ability is referred to as "Cursed Seal Transformation" (呪印化, Juinka).
While these clansmen can absorb natural energy passively without being turned into a statue, as those who failed to learn senjutsu do, their bodies cannot control the immense power surges that accompany the transformation and as such, are prone to sporadically going berserk.
Kabuto Yakushi injected himself with the biological material of Jūgo to gain this ability before traveling to Ryūchi Cave. Using the ability to absorb natural energy passively aided him in training to achieve Sage Mode under the White Snake Sage.
By absorbing a small portion of senjutsu chakra from Madara, Obito was, according to Madara, able to perform Sage Transformation. Obito's Sage Transformation left his body physically unchanged, however he gained the addition of a single Truth-Seeking Ball reshaped into the Shakujo he wielded as the Jinchūriki of the Shinju.