In this circumstance, handfasting was used as a means of temporary betrothal until a minister could make his way to the area to perform the actual religious ceremony.
The literary source for the "year and a day" originally comes from Sir Walter Scott. A year and a day was the period that a couple were to have been married in order for a spouse to have any claim of inheritable property in the event of the death of the other spouse.
If, prior to the year and a day, either party chose to leave the "marriage", the relationship was considered null as was any future claim to inheritance. Any children who had been brought forth prior to the annulment were still considered lawful offspring of both parents. Further, neither partner could be prevented from seeking marriage to another person once the handfasting was dissolved.
In a handfasting ceremony, the hands of both the bride and the groom are joined just as we see in modern marriage ceremonies today. The person officiating at the ceremony would then wrap the clasped hands in the end of his stole to symbolize the trinity of marriage; man and woman joined by God. This symbolic binding together in marriage later evolved into a the practice of wrapping the clasped hands with a cord or an embroidered cloth.
The couple were then considered to be officially bound together and could live as man and wife. Once the minister made his way to the parish or area where the couple resided, then an official church ceremony would take place, sealing the marriage.