I haven't done one, but basically downstocking is taking inventory from the back to replace what has been picked from the shelves. So you'd be moving product and needing to be very specific on what you take, how it's positioned, and your record keeping.
Best Buy is one client I frequently see these jobs for. And a web search indicated to me that downstocking is a core part of their employee training. So I'm left wondering why this is run as an ad-hoc for a shopper. Quality control? Or just outsourcing an unwanted task?