The Midrash gives a mashal to a hunter who was pursuing a bird and the bird flew off and landed on a statue of the king. if he shoots an arrow it may damage the statue. if he throws a stone he would be guilty of stoning an image of the king and forfeit his life.
So to, Bilaam cursed Amalek and was trying to curse Yisro. Yisro went and joined Klal Yisrael, and thereby escaped Bilaam's clutches.
Question is how did the fact that Yisro joined klal yisrael similar to the bird sitting on the king's statue? Yisro did not merely sit in a different spot, he changed his entire lifestyle. The bird merely sat on a statue.
Must be that as soon as Yisro decided that he is joining klal yisrael he was considered to be 'on the King's statue' and could not be harmed, even though the process of giyur was far from complete.
Similarly, someone who sincerely accepts upon himself to do teshuva, is considered as being bound to the King and is under his protection.
Sefer Bad Kodesh from Rav Boruch Dov Povarsky shlita