When you start this learning mode for the first time on a flash card set, all cards are in compartment 1. When you have 75% of all cards in compartment 2, compartment 1 and compartment 2 are queried.
If 80% of all cards are in compartment 3, compartment 1, 2 and 3 are queried. If 85% of the cards are in compartment 4, compartment 1, 2, 3 and 4 are queried.
If 90% of all cards are in compartment 5, all cards in all compartments are queried until all cards are in compartment 5.