Senjutsu

Senjutsu (仙術; Literally meaning "Sage Techniques") refers to a specialised field of techniques that allows the user to sense and then gather the Natural Energy (自然エネルギー, shizen enerugī) around a person. Senjutsu practitioners can then learn to draw the energy of nature inside of them blending it with their own chakra (created from spiritual and physical energy within the shinobi), adding a new dimension of power to the sage's chakra, resulting in the creation of "senjutsu chakra" (仙術チャクラ, senjutsu chakura). This chakra cannot be seen by anyone other than those who have been trained in senjutsu.

This new chakra enables the user to enter an empowered state called Sage Mode, which can then drastically increase the strength of all ninjutsu, genjutsu, and taijutsu. A person who is able to use senjutsu is called a. Currently, there are two known ways to learn senjutsu. One of them is to study under toads from Mount Myōboku, as Shima, Fukasaku, Jiraiya and Naruto have done. Gamakichi and Gamabunta also seem to be familiar with it. Another way is to study under snakes from the Ryūchi Cave where Kabuto Yakushi learnt it.

Usage
According to Fukasaku, only those who already possess "extreme chakra levels" can utilise natural energies to invoke senjutsu.

The user is required to balance his/her physical and spiritual energy with the natural energy he/she gathers from nature; Gamakichi compares the process to adding an extra flavour to a chocolate and vanilla swirl ice cream. If the user puts too little natural energy into using senjutsu, the technique will not work.

While gathering natural energy, the gatherer must remain absolutely still in order to be one with nature. They can use a special toad oil that allows the natural energy to enter their body, but this oil only exists in liquid form in Mount Myōboku's atmosphere. The oil evaporates if it is taken outside, thus rendering it useful only for learning purposes. If the user takes in too much natural energy, the user will start to turn into a toad and, eventually, turn to stone. The petrified remains of one-time apprentice Sages who failed to master senjutsu are spread throughout Mount Myōboku.

Getting hit by a special staff during the transformation will reverse the toad transformation by knocking all the natural energy out of the gatherer. It is dangerous to learn how to gather natural energy without the staff because there is no other known way to reverse the toad transformation. Furthermore, if the transformation is completed, not even the staff can reverse it, and the user would become a toad statue forever.

If the practitioner can achieve a perfect balance between the physical and spiritual energies of their own chakra, and the energies of nature, they can produce a new and stronger chakra, called "senjutsu chakra".

Sage Mode
Those who are able to learn to use senjutsu are able to enter a physical state called "Sage Mode". When active, Sage Mode increases the user's strength and speed, as well as augments the power of their techniques.

Jiraiya had trouble using Sage Mode, as he slightly turns into a toad whenever he uses this form. This involved his nose and eyes (although the eyes can be excluded because as Fukasaku said the "toad eyes" are always present in Sage Mode being the sign of a true Sage) changing to a more toad-like appearance. Also, he grows a goatee just like Fukasaku's and his fingers and toes are toad-like. In order to use Sage Mode in combat, Jiraiya relies on Fukasaku and Shima to gather natural energy for him.

However, Naruto, who has just learned how to sense natural energy and create sage chakra, only has reddish-orange pigment markings around his eyes and horizontal, golden slitted-pupils, showing his mastery over his Sage form. Naruto is able to use his Sage chakra very well, demonstrating the ability to lift a giant toad statue with his Sage chakra augmenting his physical strength soon after his induction to senjutsu. In order to use Sage Mode in combat, Naruto uses shadow clones to gather natural energy for him and then has them disperse to transfer their natural energy to him.

At some point after he integrated Orochimaru's DNA into his own, Kabuto Yakushi went to the White Snake Sage. Under its tutelage, he learned senjutsu which enables him to use Sage Mode. While in Sage Mode, he gains darker pigment around his eyes that are similar to a snakes, as well as four horns. His senses are also dramatically increased, to the point of being able to dodge an arrow from Sasuke's Susanoo.

Trivia

 * In the anime, the sound of natural energy being gathered to form senjutsu chakra is the same as the one used for the healing green chakra of the Mystical Palm Technique.