Ekadashi is an occasion for fasting, when one forgoes food. Nirjala Ekadasi is the day when one foregoes even water.
Ancient India recommended fasting a day in a fortnight, the designated day being Ekadasi. Many still practice Ekadasi fasting religiously with a clear understanding of the biological and therapeutic benefits of this practice. The position of the sun, moon, earth and the gravitational pull they exert on each other on this day that makes Ekadashi special.
Ekadash, eleven also stands for
1. 5 Jnanandriya – eyes, nose, ears, tongue and skin
2. 5 karmandriya – arms, legs, digestive system, excretory system and reproductive system
The 5 senses of perception and action, and the mind as the eleventh. Ekadashi is a day to bring the senses into harmony with the 11th aspect, the mind. One of the ways to do this is through fasting.
Hence fasting is undertaken during every Ekadasi. Nirjala Ekadashi is more austere, where they observe, without even drinking water.
Which is why it is called Nirjala Ekadashi.
There are four levels of fasting, based on intake
1. Eating one regular meal in the day
2. When fruits and milk are consumed
3. When only water is consumed
4. When, even water is not taken
This last category of fasting is undertaken on Nirjala Ekadasi day, when even water is shunned. Fasting once every month and annually without consuming water is prescribed as a cleansing process for the body.