Research based on the Toad King arrancar which had the ability to steal ones reatsu with its toungue.
Further study showed that the reatsu draining was caused unique, and never seen before enzymes in the hollows saliva, and that by swallowing it the arrancar would add the reatsu it had stolen to its on reastu.
preparation
The enzyme has been isolated and special cells have been developed that can secrete this enzyme. Because of this, the liquid that is formed is more concerntrated that the arrancars saliva. The cells cannot thrive in bodies and so are grown in agar's in the research department labs, the process is slow so only small amounts of the colourless reatsu draining liquid is manufactured each day, about 1 test tube.
The manufacturing is slow but the liquid works instantaneously. When one comes into contact with the liquid, dipping a finger in it for example, the person feels a draining sensation as the liquid consumes a 10th of the persons reatsu and takes on the colour of the persons reatsu, it becomes a liquified version of the shinigamis reatsu.
Usage in battle
as mentioned earlier preparation is not suitable for battle however a premade test tube can be carried into battle, this requires the ITEMS feat. by drinking the liquified reatsu the shinigami gets a tenth of their reatsu levels replenished and minor injuries like small cuts and bruises are healed
