"Introduction of The Zohar," Article “The Donkey Driver,” Item 85: …The donkey driver who is leading the donkeys behind them is the assistance to the souls of the righteous, which is sent to them from above to raise them from one degree to the next.
…Hence the Creator sends a high soul from above according to the merit and degree of each righteous, and it assists him on his way…. This is called “the impregnation of the righteous’ soul.”
Question: When do we merit the impregnation of the righteous’ soul?
Answer: It happens when we desire to ascend. There is no problem; the ladder of the degrees does not change. The forces that operate on the degrees are constant.
Everything is accessible; everything is waiting for the launch signal. And we are the ones who launch this system.
There is nothing we should be waiting for. Push the “buttons,” pull the “levers,” and everything will start working. You need to launch this mechanism. It is under your control, so go ahead!
This is why the “when” depends on a person. It is whenever you want it.
From the 2nd part of the Daily Kabbalah Lesson 1/11/11, The Zohar