I haven't heard of it until today so I had to look for it in the internet...
Maybe it is going to replace the engagement ring, because:
1) both sides of the relationship can wear it,
2) nowadays many relationships last for years without even planning the engagement or marriage.
Anyway I haven't heard of it in my country, but who knows? Many foreign customs are adopted in the times of globalisation.
Still I haven't noticed anything on my son's and his girlfriend's hands last time I saw them ;-)