An idiom that refers to the gist, heart of the matter, the point or nitty-gritty. It can also be used to describe a person or thing that is responsible for the inner workings of a larger whole.
Where the rubber meets the road
Mike: "So I went out to dinner with Jen. She was flirting all night and we went back to my place after. We started cuddling....
Dave: "Dude, where does the muscle meet the bone?"
Mike: "We fucked."
"There are many people responsible for this company's success but Jim's behind-the-scenes work is where the muscle meets the bone."