Ah interesting, it looks like you are right. If the any partition grows to > 10GB on the main table or a GSI, that partition splits into sub-partitions using the sort key as part of the hashing function. If there is no sort key, the partitioning scheme equally distributes items across partitions, so all partitions sub-divide at the same time.
https://stackoverflow.com/questions/40272600/is-there-a-dyna...