(CNBC) - What started in Buffalo as a way to help a local wine retailer has turned into a national phenomenon. Cash mobs have become a national movement to support local businesses. What's a cash mob? Think flash mob, only doing something more useful with one's time. The idea is simple: Organizers tweet to their followers to show up on a certain day at a meeting place, with cash in their pockets. Once there, cash mobbers learn what store they will be patronizing. And then, they go shopping, with the promise to spend at least $20. Store owners are warned ahead of time, so they are not overwhelmed with shoppers.