The reason this area is called a Cum Trough is because when engaging in doggy style sex or a facsimile thereof, the man may be so inclined to pull out of the woman and ejaculate on her mid back. Rather than just leaking every which way, the Cum Trough acts as a trench or trough of sorts and causes all the semen to stay in a neat straight line on the woman's back, presumably until she stands up/rolls to either side.
Bob: Yeah you're lucky, I keep going out with these damn skinny women, they have nothing but spine. Where is a man to deposit his seed, I ask you!
Bob: Oh yeah... I'll get back to you on that.