People will sign up and then pay an amount of money each week. The amount is up to the individual. At the end of the year they will get back exactly what they paid in, leaving your organization to pick up the interest. Well worth adding to your list of fundraising ideas if you've got a reasonable sized community to work with.
I often wonder why people dont just put the money in an account of their own so that they pick up the interest. What I do know is that people feel the need to save for the holiday season. I think the reason why savings schemes are so successful is that it takes the persons willpower out of the equation for them. You arrive on their doorstep and they hand over money, instead of them having to discipline themselves to go to the bank each week (the obvious temptation being that they skip some weeks to buy something they saw in the store). You should do some research to source the best possible account for your purposes. Savings schemes are fairly time intensive for your volunteers and I would only recommend them if you have enough volunteers so that each of them is looking after no more than 10-20 people. The schemes can attract people who are not very interested in your organization so thats an added bonus.
The last savings scheme we ran we made about $3,000 from a total of about $250k (collected over a year). It was a fair amount of work for what we got but I believe it would have been much easier if we were in more urban area. If your community organization is in an urban area then I would recommend it for your list.