I don't know how it is made, but at a guess it is something like an avocado, lots of condensed milk, and ice blended together; the brown streaks are chocolate condensed milk. I don't really care how it is made, but I think it is delicious. It's perfect with anything that has some spicy sambal, is a great stomach liner before something a bit oily, or is so versatile it can be a pudding at the end of the meal; highly recommended.
PS. Incredulously, the Lonely Planet Indonesia guide lists avocado juice in a list of four "We Dare You...specialities that make for a real culture shock" (together with dog in Sulawesi, durian, and Balinese pork offal specialities), which is just silly because whilst it's not everyone's cup of tea it's hardly different from very sloppy gaucomole.
