Musar do Hioba 42:19

Let us return to my earlier statement that ההולך בלא בנים מת הוא בעולם הזה ולא יחיה בעולם הבא, "anyone who is childless is considered dead in this word and will not live in the World to Come." There is a way to overcome this handicap posthumously by means of the levirate marriage. The first thing one has to bear in mind is that every husband deposits part of his רוח, spirit, during marital relations with his wife. This is so because he has only acquired the seventh part of his צורה or דמות by union with her. The spirit he has deposited within her remains within his wife even after he has died, especially during the first twelve months after his death. All this has been explained by the Zohar on פרשת משפטים where the subject is discussed. This residual spirit of her deceased husband is at war with the second husband whose spirit is also deposited within her by their union in marriage. During the struggle between these two spirits either one may emerge victorious on different occasions.