Contest rules contain a lot of legalize and restrictions, and hidden inside that mess is this gem:
Any awarded prizes will be provided to a winner only after the winner has signed a Winner's Declaration and Release. Sponsor's current Winner's Declaration and Release for the Contest may be obtained upon request by contacting Sponsor by email at [service@instructables.com] or by mail at the address identified in Section B.18 ("Winner's List; Mailing List") below.
I.e. if you don't pull that mysterious Declaration and Release out of their ass in advance they will surprise you with a fun form and a threat "Sign this, or you won't receive your prize" later.