The Last Airbender (Avatar) has characters in it who can bend (telekinetically control) the four classic elements using martial arts movements and their minds. Part of the show includes blurring the lines between the elements, such as earth vs metal, or in this case, water and blood.
Katara, the girl in this image, is using her powers to 'bloodbend' Aang's dick to be hard before he would normally be capable of. It's a joke in the similar style the one i attached to this post.
