Hama bloodbending an elephant rat
Bloodbending is a special ability possessed by Waterbenders. This skill allows the waterbenders to control of the motions of fluids within an organism's body. This sub-ability is known as one of the most dangerous of all the bending arts. Most waterbenders are only able to bloodbend during a full moon due to it's effect on the waterbenders (strengthens and amplifies their abilities) but few are capable to use this skill at any time at will
This special technique was discovered by a skilled and young waterbender, Hama, who was locked up by the Fire Nation in a prison that is made for only waterbenders. During her imprisonment, she realised that every living organism contains water and at every full moon, where it strengthens and amplifies her abilities, she would practice on elephant rats. She eventually mastered her skill and used her blood bending ability on the guards and broke free.
Although this skill is possessed by waterbenders, the form of bloodbending is rather different to waterbending forms. Waterbending has more of flowing sort of movements while bloodbending has more sharp and quick movements and is quite similar to a pupeteer's movement when using a marionette. Yakone and Amon are so far, the only waterbenders who are able to bloodbend without any movement, this is known as "psychic bloodbending".
 The Puppetmaster
 Forms and Styles
 Common bloodbending
 Psychic bloodbending
Psychic bloodbending is where the bender is able to bloodbend without making any movement or motion with their arms or legs. Yakone seems to be the first bloodbender to develop this ability. He has demonstrated this when he was on trial and effortlessly bloodbended every person in the courtroom so he could escape. When he discovered both of his sons, Noatak and Tarrlok, are waterbenders, he passed down his knowledge. Though, only Noatak mastered the ability at the age of fourteen.
 Bending severing
Bloodbenders have the potential to cut off a bender from their bending abilities, this is known as bending severing. So far, Amon seems to be the only bender who has demonstrated this skill of bloodbending. The user must have direct contact to the victim's forehead with their hand(s). The effect is long term and permanent unless somebody is able to restore the victim's connection to their chi pathways. Any methods of healing is ineffective and only energybending can restore the bender's connection to their bending abilities.
Despite of the fact that Avatar Korra's bending abilities were "removed" by Amon, she was actually able to airbend after she found her connection to it because when Amon did "bending severing" she only had three activie connections to bending (water, fire and earth) and orinigally had absolute no connection to airbending whatso ever. When Katara, the best healer alive, failed to restore Korra's bending abilities, Korra feel to her lowest point and then found her connection to her spiritual self and Avatar Aang demonstracted to her how to use energy bending to restore the connection a bender (including herself) to their abilities. Korra then used energy bending to restore other people's connection to their bending abilities.