The scheme is based on a recursive and deterministic mathematical function that traverses ("crawls") the ledger, mutating a global random number with each iteration. Entropy is provided by transactions made on the ledger that approximate perfect randomness, as human transaction activity is not predictable. The sequence can to a large extent be predicted in advance.
Use-cases include selecting validators in proof-of-stake (or proof-of-vote, game theoretically nearly identical to proof-of-stake. )