The marriage vows taken in the handfasting ceremony may be for "a year and a day," "a lifetime", "for all of eternity" or "for as long as love shall last." Its very optional as it also promote a sense of free soul. Nonetheless, it also heavily promote a vows renewal, so as to enhance 'the burning of love' carryon.
Origin of the term
The verb to handfast in the sense of "to formally promise, to make a contract", especially in use of the contract of marriage. The derived handfasting is for a ceremony of engagement or betrothal; recorded in Early Modern English. The term was presumably loaned and used in Middle to Early Modern English for the making of a contract; in direct translation "to strike a bargain by joining hands".
<Harry Potter & the Half Blood Prince (The Movie)>
----Snape & The Unbreakable Vow Scene----
Source: Youtube
It depicts a handfasting ritual in a contract term (non-marriage related).
The lines you saw glowing on their hand reflect the ribbon/rope use in the handfasting ritual.
In Simple Term
Handfasting > The blessed marriage rite in which the hands of you and your beloved are wrapped in ribbon as you 'tie the knot'.
<Charmed (TV Series)>
----Piper and Leo's Handfasting Marriage Ritual Scene----
Source: Youtube
----A Real Handfasting Marriage Ritual----
Source: Youtube
U may try this with your lover!!
And may your love LIVE ON!!!! (^0^)
