QOUTE:

Absorption of Another Soul: Ibbur by Mark Elber

The word ibbur was originally used interchangeably with gilgul as a term for transmigration of the soul. Over time it came to have a separate, but related, meaning. Ibbur implies “transferring” and the word is also used to mean “conception.” The same root is used for pregnancy, fetus, and related words.

As the meaning of ibbur separated from gilgul, it came to refer to a soul that inhabits another body for a limited period of time, as in a pregnancy, in contrast to a gilgul, in which a soul is in a new body for the life of that body. Usually there were two purposes that Kabbalists attributed to an ibbur temporarily entering a body. One was to fulfill some mitzvot or tasks that the soul had not completed in its previous lifetime. The other was to help a particular person to carry out a task for which this visiting soul was particularly known.

Though an ibbur is a temporary resident, the length of time the additional soul may inhabit another's body could last years. Primarily the ibbur was there for positive reasons, to bring strength, goodness, and aid in the performance of mitzvot.