After the first restriction in each Partzuf that begins to work with the Upper Light, there is a head (Rosh) that calculates how much Light can be received in order to bestow, and then a body (Guf) receives that Light in practice, according to the decision of the head.
The head is the main part, while the body only carries out its decisions. This how it has always been, except for one time when the body suddenly didn’t carry out the decision made by the head; this happened at the time of the shattering of the vessels. The head wanted to receive all the Light in order to bestow, while the body wasn’t able to do so.
The shattering of the vessels is a state of incompatibility between the head and the body, not the action of the shattering. In the spiritual world, everything is determined according to the quality. A shattered vessel is a vessel that cannot prepare itself in advance to match the body. It isn’t the actual vessel that is shattered but the connection between the desire to receive and the thought. We can work out great plans in our thoughts, but when we wish to fulfill them, it turns out that it is impossible to carry them out.
From the 3rd part of the Daily Kabbalah Lesson 3/23/14, Writings of Baal HaSulam