-Egypt for a time, was the world center of occult; it was the gathering place of the mightiest sorcerers.
-Using all their powers, they cast a spell preventing any slave from escaping Egypt.
-They set up statues on all their borders. These statues had been created with the power to alert them of any escaping slaves.
-They were so confident in their ability to stop slaves that they made a law that who ever did manage to get past the statues would officially be free.
-This is why the area in front of Bal Tzefhone (one of these statues) was called Pei Hacherot (mouth of freedom).
-This is also one of the elements of the miracle that Yisroel was able to leave Egypt.
-This is also part of the reason why Paroah was so stubborn. He saw that the Jews did not take the opportunity to escape during the Plague of Darkness when all the Egyptians were frozen in place. He thought that that the spell was still in place and they were unable to leave without permission.

(Rabbi Yonasan, Tiferet Yonasan – Bshalach)