According to Rowena MacLeod, the curse is designed to cause a person to forget themselves. This starts off as periods of memory loss that grow more and more severe until the target has no memory at all. The curse will eventually make them forget how to talk, swallow and breathe, ultimately resulting in their death.
In 2017, Gideon Loughlin used the memory curse on hunter Dean Winchester after Dean mortally wounded him with a witch-killing bullet. Gideon died shortly afterwards while Dean retained no memory of the night leading up to being cursed. As Dean's memory loss worsened, his brother Sam turned to witch Rowena MacLeod for help. After realizing that the Loughlins' were responsible, Rowena informed Sam they needed the Loughlins' Black Grimoire to break the curse. After the Loughlins' were killed, Rowena was able to break the curse and Dean's memory from before he was cursed returned. However, he possessed no memory of what happened while cursed.
To curse a target, the witch must draw a druidic sigil in their own blood. Once the target is in range, the witch chants: "Dearmad", which is Irish for "forget". The symbol then envelops the target in purple light and the curse takes effect.
The reversal spell is unknown, but results in a similar purple light engulfing the cursed person and the witch chanting a spell. Afterwards, the cursed person's memory returns from before they were cursed, but they retain no memory of what happened while they were cursed.