In fact there is no real secret as sages wanted to reveal everything. In order for us to hear what they said however we need to learn it ourselves through trial and error.
In order to take the study of the Zohar to the next level, it must be read with an inner concentration that can be defined only as intention.
The difficulty of explaining what an intention is lies in the fact that the spiritual world has no counterpart in our world. Even if the object of our studies becomes clear, our understanding of it is only temporary. What we learn is grasped by the spiritual component of our ability to understand, which is constantly renewed from Above.
Thus, a subject we initially understand may appear unclear at a later date. Depending on our mood and our spiritual state, the text can appear as either full of deep meaning, or entirely meaningless.
Do not despair if what was so clear yesterday becomes very confusing today. Do not give up if the text appears to be vague, strange, or illogical. The Book of Zohar is not studied for the sake of acquiring theoretical knowledge, but to help us see and perceive what is hidden from us.