Sometimes it is, sometimes is like a weird ass "life magic core" that if messed up fucks up the link between the life magic that keeps a yukkuri alive and it's body. (basically what prevents it from being an average manjuu/dumpling)
Though being honest, considering how yukkuris turned out like, It seems that all the life magic used on them was wasted...
For Origin story there was also this anko on which it was explained yukkuris were like the prinnies of disgaea, basically Eiki stuffed the most sinful and wicked souls on them and throw them back to earth so they would live the miserable life yukkuris have. Reason why on that story Eiki approved of abyuse since no soul used on a yukkuri was good (reason why yukkuris act as bad as we know).
Personally it's not my cup of tea since that origin would mean no yukkuri is good and that we have to bear with them because Eiki's hell is full, so all that stress and crap humans face because of the yu-antics would be product of afterlife bureaucracy.
In all seriousness, looking for Origin stories and making a summary or translate them would be an interesting alternative to the typical abyuse stories or the typical cute stories.
Gib back... Marija's...
important bean pashte...
Marija... will die...!