It will make the release speed up when there is more range (to increase gain) left in the band's AGC. The more range there is left, the faster it will speed it up. That control lets you set how much of that effect you want to have.
You should download the manual for the Omnia.9 if you want to know what some more of those do. 🙂 Most of them you will know what they are when reading the manual. Of course there any many things the Omnia.9 has, which BreakawayOne doesn't have, but it is still very useful.