Senjutsu (meaning "Sage Techniques") refers to specialized field of jutsu that allows the user to sense and then gather the "natural energy" around a person.
Senjutsu practitioners can then leave 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 dimension of power to the sage's chakra, resulting in the creation of "senjutsu chakra". 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 ninjutsu, genjutsu and taijutsu. A person who is able to used senjutsu is called a sage.
According to Fukasaku, only those who already possess "extreme chakra levels" can utilize 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 flavor to a chocolate and vanilla swirl ice cream. If the user puts too little natural energy into using senjutsu, the technique will not initiate.
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 "Sage Chakra".