The "Breed 20 Eggs For Guaranteed Progress" feature

How does the "Breed 20 Eggs For Guaranteed Progress" feature work?

When you spend rubies to breed 20 eggs, the guaranteed progress means that you will at least get one shard of one undiscovered egg - not one shard of all undiscovered eggs. For example, if you are breeding Volos and Baldr to try get Amarok eggs, but you do not yet have Noss either, you are guaranteed to get either a Noss or Amarok fragment, not both (although you could get both or multiple fragments of each if you're lucky).

One good tactic is to breed a pair of dragons that has all discovered eggs except the one you want.