This new chakra enables the user to enter a powered state called Sennin 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 Sennin. It would seem that it stems from the toads of Mt. Myoboku.
Toad Transformation Edit
Only those who possess a high chakra level can utilize Senjutsu. The user is required to balance his physical and spiritual energy with the natural energy he gathers from nature. If the user puts too little natural energy into using Senjutsu, the technique will not initiate. If the user puts too much natural energy into the techniques, the user will start to turn into a toad (Jiraiya had almost turned into one). Furthermore, while gathering natural energy, the gatherer must remain absolutely still in order to be one with nature.
Getting hit by a special scepter 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.
Jiraiya had trouble using the Sennin 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 Sennin Mode being the sign of a true Sennnin) changing to a more toad-like appearance. Also, he grows a goatee just like Fukasaku's and his fingers and toes are toad-like.
However, Naruto, who has just learned how to sense natural energy and create Sennin Chakra, only has red rings around his eyes and horizontal, golden slitted-pupils, showing his mastery over his Sennin Mode (Naruto's mastery of his Sennin Form may be owed to prior experiences in Jinchuriki Form). He is able to use his Sennin Chakra very well. He was able to lift a giant toad statue with his Sennin Chakra augmenting his physical strength.
Before Naruto was able to utilize the Sennin Chakra correctly and evenly, he used frog oil, which allowed the natural energy to enter his body. However, the oil involved can only exist in liquid form in the Mt. Myoboku's atmosphere, and evaporates if it is taken outside, thus forcing Naruto to gather natural energy on his own. Jiraiya was able to continuously draw in natural energy by having Fukasaku and Shima remain motionless on his shoulder while he fought, but the Kyubi no Yoko (Nine-Tailed Demon Fox) rendered this impossible for Naruto.
In order to work around this problem, Naruto had two of his clones gather natural energy, and dispelled one when he ran out, gaining the clones' Sennin Chakra. Furthermore, usage of high-level techniques while in Sennin Mode can hasten the exhaustion of the user's Sennin Chakra reserves, as was seen when Naruto quickly reverted to normal after he fired a Futon • Rasen Shuriken at Pain's Human path and a Senpo • Rasenrengan at Pain's Animal Path.