HATHA Yoga is described in the yoga upanishads as a means to attain  physical and mental purification and balance. 

In order to purify the mind it is necessary tfor the body as a whole to undergo a process of absolute purification.

So thats why hatha yoga is also known as the science of purification, not just one type of purification but six types.

The body has to be cleaned in six different ways for six different impurities.When you clear the body of these impurities the nadies function and the energy blocks are released .Then the energy moves like waves frquences throughout the channels within the physical structue moving right upto the brain

Hatha Yoga is preliminary practice of tantra, which is to be studied before one practices Raj Yoga, Kundlini Toga & kriya Yoga.

The main of hatha yoga is to create an interaction of mind and energy. When this harmony is created, the impulses generated give a call of awakening to the central force, shusumna nadi which is responsible for the evolution of human consciousness.

HA and tha is combination of 2 beej mantra, ha represents prana, the vital force and tha represents mind the mental energy

When union between pranic and mental forces take place then a great event occurs in man This is the awakening of higher conscious ness or spiritual awakening.

Hathayoga is divided inti six traditional shatkarma   NETI, DHOUTI, BASTI,  NAULI, KAPALBHATI, TRATAKA.