The belle is referring to the woman's clitorous whilst the taco refers to her reproductive organ. The belle hangs in the taco just like the woman's sexual organ happens to appear.
Dude have you seen Carly's taco belle, it's bigger than what i expected when i saw it in the mirror as she was running after me
(N.) A girl that will ask you to go down on her at 2am. There is usually too much cheese, and even though you said "no sour cream" there's a huge glob of it. The "shell" is usually soggy, and overloaded with "lettuce."
"I went out with this girl, Sabrina. When we got back to my place she was such a Taco Belle. I didn't stopped throwing up for a week."