The Kundalini-force is the force which stands very near to the material plane and to the human body.
Kula -Kundalini : In the various systems of yoga this force is symbolized as a fiery snake which sleeps in the center of the lowest chakram. It is the final aim of these scools to awaken this force and to transform it at the same time while it rises up to the highest chakram.
Some directions of Kundalini-Yoga try to awaken this snake through direct concentration on the middle of the lowest chakram with support of certain mantrams. But the result is very often an unbalance of this power, because the other chakrams are not enough developed to transform the gained energy. Then this negative power overfloods the yogin who becomes a slave of the lower energies.
Kundalini is a snake, and she is a negative and a very gross force !
First throught the transformnation this energy can be the material of the inner stove for the higher forces.
The traditional path of Kundalini-Yoga is described as the "lower path" , the tree of the recognitoion of good and evil, which is nourished by the transformation of the lower energies.
The "upper path" brings the higher energies down and transforms the lower energies but does not stimulate them ( Tree of life ). Therefore here the chakrams turn around in the opposite direction. (Chakra has the meaning "whirl of energy")
Examples of such scools are Vajrayana - Kagyu or the Kriya-Yoga of Yoganada.
Finally there is the pure spiritual path which is not for our negative world but mostly for the life around a true master. Hinduism sees such high spiritual techniques as adequate for the Satya-Yuga.
Prana-Kundalini : Before the fiery kundalini can arise the forces corresponding to the Prana-Maya-Kosha of the chakra above must be cleaned(common Chi or lower prana). This is for example a theme of the Yoga Kundalini-Upanishad.
Therefore many modern Kundalini-Scools do not practise the awakening of the Kundalini but are in reality tantric scools with meditation and prana-exercises.
Gopi Krishna described in his book how dangerous an uncontrolled awakened Kundalini can become.
A Wiki about the Kundalini