A beautiful term, for a beautiful thing. Gently put, when one has a lot of Mexican Food the previous day, and takes laxitive after eating the Mexican Food, the person will have an enormous amount of anal discharge the following day. Now this is where the chocolate waterfall part comes in. After the person completes the anal discharge, he or she is suddenly called out of the bathroom, and is unable to use any toilet paper to remove the leftover anal discharge remnants. Then the person walks outside and goes for a nice walk by the road in 100+ degree weather causing the person to experience a large amount of sweat. Then, the high amount of sweat combines with the leftover anal discharge, which is termed Bukanga (Japanese for "Love Mixture") and that will drip down his or her legs resulting is the... Chocolate Waterfall.
When I saw the Bukanga going down her leg, I was thoroughly disgusted and ran over to the nearest trashcan and threw up.