Illusion forms if a truth or a principle in your mind is mixed with self-interest, selfish plans, and goals. Thus truth is taken from its purity and mixed with your ever-changing self-image and self-projections.
Maya is the crystallization of the forces in your etheric body that prevent perfect circulation of energy, and you feel weak. It happens when certain energies and forces come through your other bodies to actualize. Before actualization they meet the resistance of your habits -- primarily sexual habits.
The Art of Teaching, p.424