It's more like a standard theurgic contract to me, honestly, except for the part where I'm not sure we still have our souls.

That aside, it uses our emotions as fuel to undergo rapid aging to a point in their life cycle where they are far more powerful, and apparently there is a direct relationship between age and power with no problems due to age. Mortal peril seems to trigger the evolutions easiest, but doing so recklessly or with negative emotions who defines that, anyway can lead to a berserk transformation, where the partner Digimon loses its mind and tries to kill everything around it, up to and including its partner.