How do you erase certain parts of the memory?

[graykin]

I, personally, find it hard to believe that you can just...snap your fingers and make someone forget something. The human brain is just too complex for that kind of thing. We are not like computers where you can just go to the Recycle Bin and hit "empty". I honestly don't care if hypnosis is real or not...but the manipulation of one's memories through repetitive hand waving and saying "You will forget having played the game Penumbra!" is just impossible...unless someone can tell me exactly how it works.

EDIT: I'm not trying to be a pain in the neck or piss anyone off. I just want someone to definitively explain to me how it works.