very similar to PoW difficulty across the bitcoin mining protocol, difficulty ramps up or down based on a metric, which is time based in the case of bitcoin. for SPL it would be matching a substring, in particular the end of the ranked battle ID, to the difficulty string. So matching 'a' would be much easier than matching 'azircon'
RE: Proof-of-Work Inspired Ranked Play Incentives for Splinterlands