Look at the back of the GameCube. If there is a "digital out" port, they were intended for the HD cable which Nintendo made 150 of and never sold them to the general public. At least that's what you think, given the crazy prices for them on eBay.

Not all games were optimized to use it, though. Some games that support the progressive scan: F-Zero, Metroid Primes, PSO. If you look on the back of the box, there's a Progressive Scan icon and all that. Easy.