I found a article about "yaa dong" in the internet:
......."when Thai yaa dong is sold, it is offered in little cups called thuay talai that holds at most 50 cc. One cup - no more - is drunk before a meal. It makes the food taste better, and stimulates the appetite, too, so that people eat more. "Later, people also began to claim that yaa dong enhance sexual performance. It was said to produce unfailing and long-lasting erections, that it was so potent that it could make a monk leap over the temple wall to head out in search of romance. ............
The whole article (and more) you find under:
http://www.snapoo.com/out/th/yaadong.html