The Kundalini-force is the force which stands very near to the material plane and to the human body. There are two different Kundalinis.
1. Kula -Kundalini : The various systems of yoga symbolize this force as a fiery snake which sleeps in the center of the lowest chakra. It is the final aim of these scools to awaken this force and to let it rise up to the highest chakram through various knots.
Some directions of Kundalini-Yoga try to awaken this snake by direct concentration on the middle of the lowest chakram with support of certain mantrams. But the result is very often an unbalance, because the other chakrams are not enough developed to transform the gained energy. Then then untransformed forces of the Muladhara-Chakra overflood the yogin, who becomes a victim of the lower energies.
Gopi Krishna described in his book how dangerous an uncontrolled awakened fiery Kundalini can become.
Esoteric snakes are negative forces ! First through the transformnation can these energy be the material of the inner stove for the higher forces.
2. Prana-Kundalini : Before the fiery kundalini can arise the forces corresponding to the Prana-Maya-Kosha of the Swadisthana - chakra must be cleaned(common Chi or lower prana). This is for example a theme of the Yoga Kundalini-Upanishad, and this happens also within the pranayama stage of ashtanga yoga.
Therefore many modern Kundalini-Scools do not practise the awakening of the Kundalini but are in reality tantric scools with meditation and prana-exercises.
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.
A Wiki about the Kundalini