[Added November 2011]
The fast answer is : exactly what are doing the “virgins” during bunga-bunga parties in Berlusconi’s (real) palaces. For the long answer please read the
book Moonwalking with Einstein
by Joshua Foer. I try to summarize (of course mistakes are mine). This book is
all about method of loci. This method has been invented around 3000 years ago and is based on two important principles that allow the fast and permanent remembering of almost everything.
The first principle is the use of “loci” (places in latin) and is
based on the fact that our brain is very good at remembering places. JF describes this very well: if you are left alone in a house never seen before
for a short time (for example an hour), that hour of exploration is enough
to memorise hundreds of details of the house forever.
Now, if you only could read a book about a complex subject or a deck of cards in the same way as you “read” the house , you have discovered a way to memorize instantly everything. This is what the second principle of the method addresses: “images agentes” vivid images of “something” happening in the places
that you already know very well (i.e. the loci).
Now, what are the things happening around you that you best remember: mostly things where other people is involved and having emotional content.
For example, if we are in a situation which is dangerous for us , we store instantly everything happening around us and we automatically replay the scene
day and night (in our dreams) for many days until that scene becomes a permanent
part of our brain. This is what means “vivid images”: imagine a scene happening in one of the loci in which someone that you know well is doing something nasty, fearsome or simply funny or bizarre. Then this scene will stick effortlessly to your brain and , if it encodes a concept in a book or a card in a deck, this information would also be stored forever. Before you rush using the
method , be warned : you need a year of training to set up enough memory palaces (i.e. empty houses) and then to create effortlessly new images…
Now coming back to the virgins… Sexual images are of course among the more memorable so Peter of Ravenna, a 15th-century writer on the topic advised “If you wish to remember quickly, dispose the images of the most beautiful virgins into memory places”.