Just brought a M&P 9 home and was wondering about this little thing. The S&W manual does a poor job in explaining this lever. They don't tell you what it does or what it's purpose is. Since I consider myself fairly intelligent, this thing doesn't really do anything at all. I re-assembled it and broke it down the same way as a Sigma--and it works the same. Plus, on my M&P, getting the frame tool out requires some tough fingers.