IAC 1887, What is the CAS Turn Down at Heel - 4 Degrees?

[mcgiiver]

Can anyone explain exactly what this modification is and what benefit it has?

[RRio]

Supposedly gives a better anchor into the shoulder and helps spread the recoil more evenly, from what I have been told. I really don't know if it would be that much noticable.
My IAC with the standard butt does not bother me.