This feature is designed to allow for a more consistent hashrate on the nicehash/westhash pools.  If you don't mine on those two pools, you don't need to turn it on.

If you do mine there, upgrade to the latest version of firmware and turn on the "2" flag--you should find a green check by the "extranonce subscription" field to the right of your miner on the dashboard at nicehash/westhash.