The very first digidestined used the ancient Digimon art of Armor Digivolving to help their Digimon. We see this in Season 2. The information from the 8 Digidestined in Season 1 was transfered to the Digieggs at the same time it was sent to the tags and crests. This is why When the Digieggs are used you see the Digimon from season 1.
Example: Tai owns the Crest of Courage, thereforewhen Davis uses the Digiegg of Courage Veemon gets Agumon (and his Digivolution's) power.
As for the other digieggs; Ken had the crest of kindness so this info. was passed on to the Digiegg of Kindness (but since this was hidden in the Digiegg of Miracles we never see this).
The Golden Armor digieggs are more powerful and can be released when digimon reach mega level.