Hairtrigger: It's part of the female anatomy region. You can learn more about it on wiki:
Rectouterine pouch - Wikipedia, the free encyclopedia
I'm assuming you want to reach the CDS with your penis? If so, there's been some mixed reviews. Several women report that getting their CDS touched/thumped hurts. On the other hand, I've heard of just as many report that it provides amazing orgasms. Either way, it's fairly deep in the vagina for most women (not all).