When enabled this will tell HireFire to retain the job/worker ratio during every checkup. When disabled this will tell HireFire to never scale down to the minimum configured dyno quantity unless all the jobs in the queue have been processed.
Make sure that you have implemented a way to gracefully handle Heroku’s SIGTERM signals. It is recommended to always do this, regardless of whether you have decrementable enabled or not, because Heroku restarts your dynos at least once per day.
Some worker libraries probably already handle this for you. Others might not. Consult your worker library's documentation.