I've run rs.stepDown(long, int) command several times with different integer paramter.
But looks like period to elect new primary is nothing to do with secondaryCatchUpPeriodSecs and replSetStepDown parameters.

And one more thing is that primary/secondary swithing by changing member priority value doesn't waiting replication sync so this method make rollback but rs.stepDown is not.
Is there any difference between above two method ? (I can't find any difference of two method in manual.)