As far as I understand it (and I'd be happy to be proven wrong) a Bausparvertrag works as long as people join the bandwagon so that people on top of the pyramid can cash out. Some might call it a Ponzi scheme, and indeed if people stop paying into it the whole house of cards collapses.

So everyone in it has the interest to bring more people in, to secure their payout.

Long story short, unless you get to understand all the intricacies, don't touch one with a ten foot pole considering how cheap mortgages are right now.