Occurs during sexual intercourse, when the male's penis is at its apex of stiffness, length and girth and as far as it can go into the female's vagina, the male then sticks his finger into the female's asshole. Hence, The Half Slab, Combo Chicken Breast.
Male - Whispers in the female's ear "That's the Full Slab" And sticks his finger into her asshole.
Female - Replies in reaction to the gesture. "No, that's the Half Slab, Combo Chicken Breast."