Senjutsu

refers to a specialised field of techniques that allows the user to sense and then gather the around a person.

Overview
Senjutsu practitioners 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). This adds a new dimension of power to the sage's chakra, resulting in the creation of. 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 being to learn from the toads of Mount Myōboku as Jiraiya and Naruto have done, and the other from the snakes from the Ryūchi Cave where Orochimaru and Kabuto Yakushi learned it. The cursed seals that Orochimaru has used in the series also use senjutsu chakra.

A third form of senjutsu is also known, used by Hashirama Senju. Unlike previous methods, Hashirama doesn't display any animal traits. It is unknown where he obtained his senjutsu style.

Training
According to Fukasaku, only those who already possess "extreme chakra levels" can utilise natural energy 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 body, Kabuto Yakushi went to the White Snake Sage. Under its tutelage, he learned senjutsu which enables him to use Sage Mode, which Orochimaru had also done in the past. While in Sage Mode, he gains darker pigment around his eyes that are similar to a snake's, 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. Combined with his research and duplication on Jūgo clan's ability to passively absorb natural energy, Kabuto can gather natural energy indefinitely even while moving in combat.

Hashirama Senju can also use Sage Mode. In this mode, he retains all of his normal physical characteristics except for a dark pigmentation both around, and underneath his eyes, a dark, circular marking also appears on his forehead. Hashirama's abilities were drastically increased. This was witnessed through the use of his Sage Art: Wood Release: True Several Thousand Hands technique which created a gargantuan, many-handed structure which easily dwarfed several mountain ranges, as well as Kurama. Hashirama can also perform Sage Art: Gate of the Great God, a technique that can bind a creature as powerful as the Ten-Tails. Like the toad and snake-taught Sage Modes, the user has to stay still for a certain amount of time so they can gather enough natural energy to form the special senjutsu chakra. Thus, the user is left vulnerable while doing this.

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.
 * The word uses the same hiragana: せんじゅつ, as the Japanese art of, possibly an allusion towards the abilities of Sages to sense persons while in Sage Mode.