I got this lovely fat quarter bundle in the post yesterday!
What happens is that someone (anyone) puts forward a suggestion for something they'd like to buy. This then becomes a 'poll' that anyone can vote on. Voting doesn't commit you to buying, but it helps the site owners work out what would be of interest to their customers. They then go off and contact the manufacturer and ask them if they'd like to offer a special deal. The manufacturer offers a deal so there is a start price if one person buys, then some percentage less if, say, 20 people buy, then another discount if, say, 50 people buy and so on. You can see current votes here and current drops here. Although there are a tonne of quilting things you also get lots of audio equipment, shaving apparatus, high end pens, geek chic and pretty much anything else people might desire.
I am not affiliated with Massdrop, except as a customer, but they do have one of those "invite your friends" links so if you want to have a look and see what's what then click here to register and if five of you do, and then buy something, I get something. Not sure what so don't feel obliged! But I have been very happy with their service and will definitely be joining in future drops.