It is not known how this Wesen removes the memories. Only that it can. I surmise it has something to do with electrical charges in the brain. After an attack, the victims are usually diagnosed with advanced dementia.
From interviewing some of these unfortunate souls, I can see why -- this is one of the most heinous Wesen I have ever encountered.
They rob the victims of their very lives by stealing their past and leaving them void of any human connection. Devastating to their loved ones and leaving the victim unable to identify the attack or the attacker. Justice within the confines of the law is therefore impossible.
I witnessed this Wesen attack a woman -- four of his tentacles pierced the back of her skull.