muntdefems: Do they meet, say, once a week and decide what's dangerous for the German youth and what isn't anymore? :P
If I'm not mistaken, anyone can submit an application for the indexing of a particular game, record, movie or book to the BPJM, who will then examine the medium in question and decide whether or not it's actually "harmful to minors".
How exactly they come to those decisions is anyone's guess, as the whole process is rather nebulous.
Additionally, an indexing normally only lasts 25 years, unless it's renewed. If a company wants to get something off the list prematurely, they also have to make an application for that, pay a fee and be very persuasive.