A reverse-key index is B*Tree index where the bytes of each indexed column
stored in a reverse order. They are very useful for Oracle Parallel Server
where contention for the same leaf blocks during data inserts can cause
inter-instance "pinging", excessive I/O and performance degradation.

Reversing index keys allows insertions to be distributed evenly across
all the leaf keys in the index.

But reverse-key indexes do not allow index range scan since lexically
adjacent keys are not stored next to each other, so you probably should not
use reverse-key indexes unless you are in OPS environment.