This was a really simple addition to the code; maybe 6 lines. But the effect was quite dramatic. AUTO, RTL, and LOITER now sense if the plane is in a dive and enter a simple stability mode for 4 seconds. Enough time to recover from the worst dive.
I fly an EZ Star and this has always been my biggest safety concern and need to flip to manual control. Now I don't need to.
In the video you can see me put the plane into a dive. This can happen when the plane turns too hard and the GPS refresh can't keep up. Once the plane begins to stall, it drops like a rock.
Update is on the SVN in Ardupilot 2.7 for Beta testers.