Angel was meant to suffer. Not to live as human. One moment of true
happiness, of contentment, one moment where the soul that we restored
no longer plagues his thoughts, and that soul is taken from him."
"Vengeance demands that his pain be eternal, as ours is. If this -
this girl - brings him even one minute of happiness. That is one
minute too many."
- Enyos of the Kalderash
The ritual is intended to curse a vampire by re-ensouling him or her, thus forcing the vampire to accept massive amounts of remorse, guilt and torment for all the lives that they have taken as a soulless beast. As far as is aware, Angel is the only vampire who has been re-ensouled in this way, and it is unclear whether other soulless demons can be re-ensouled in this way.
Restoring a vampire's soul in this way is not intended as a gift. It is very much a form of revenge, or a curse since if ever there comes a time when the restored soul no longer torments the vampire, the moment the vampire is not plagued by the guilt of what they had done for the sake of evil in the past, then in that instant the soul is taken from them. Angel's soul has been lost this way twice.
Whenever the curse takes effect, it takes a few moments for the vampire to remember their actions while soulless, and when it is broken, the vampire suffers debilitating pain as his soul is ripped from his body.
While the Clan Kalderash intended the ritual as a curse, when the Scooby Gang and later Angel Investigations had the ritual used on Angel again, it was intended merely to restore Angel's soul and bring back Angel rather than Angelus. This is in contrast to the intentions of the Clan who wanted Angel to suffer because of the ritual.