Retec provides Sainsburys with its ‘Entertainment Xtra’ display stands - these incorporate both interactive and looping media screens that show customer DVD previews, as well as allowing them to select different trailers from the back catelogue. Retec produces specially adapted DVD promos (going through the classification process to ensure that all trailers are suitable for a family audience) and Sainsbury's sells screen advertising space to the DVD distributors.
Granted we haven't seen the data yet (and we probably never will), but we have to suspect that that 24% number wasn't an average, it was probably an overall number, or a maximum. That is to say, it's highly unlikely that the signs were able to lift the sales of every DVD on the rack by 24% (or anything close). More likely, the screens drew awareness to the fact that the DVDs were on sale at all, which led to large boosts for the big blockbusters, and maybe some more modest gains for the others.