The Strengthening Prescription: Chakra Injection was a dark medical ninjutsu that Yomi's clan had spent generations to perfect just for the night of releasing Mōryō's spirit in the Land of Demons.
This technique creates dark snakes that forcibly connect with a person and inject them with medically altered chakra. This extra chakra will allow the user to utilise high level element ninjutsu. It also allows the user to use elemental techniques that they wouldn't be able to master normally. The chakra is not infinite, and must be replenished over time. Furthermore, only one type of elemental chakra can be injected into the person at a time, as Neji noted that Shizuku was seen with a completely different element from the previous encounter.
The person injected with these snakes can also force the snake out, and drink the chakra directly, vastly increasing their chakra and power, as well as inducing a body-mutating transformation (in Gitai's case, he became an Asura-like entity, with three faces, six-arms and a tail), but results in their body being highly unstable, which, if struck with sufficient force, can result in the body exploding.
Yomi claimed that Kusuna is the only one amongst the Gang of Four who could control the chakra snakes.
When first used by Yomi, the chakra snakes emerged from the ground as long, glowing tubes that simply injected the Gang of Four with chakra, and then retracted. However, when used by Kusuna afterwards, they are shown as snakes that enter the other three's bodies via the back of the neck, which gave them a sense of exhilaration upon injection, and they gain markings over their bodies and, in Shizuku's case, her elbow extends.