Credit:
The site reads:
"When you purchase select DVD and Blu-ray titles from Amazon.com you will also receive an Amazon Video On Demand standard definition version as a gift with purchase. You’ll be able to instantly enjoy the video on demand version on your Mac, PC, compatible device or compatible TV when it is available."
The downsides: Disc+ on Demand is only available on a very limited number of titles right now, and Amazon is promoting it as a "limited time offer" -- we fully expect that demand will be high, and that the service could (and should) become a permanent one.