The joke is as follows:
有一个小伙子，去一家外企应聘，面试结束后，乘电梯下楼。电梯在中途某一层停下了，门打开后，小伙子看见一个衣着性感的女郎，身体斜靠着电梯门，用挑逗的语气问：够淫荡吧？小伙子很幽默，回答说：淫荡是淫荡了点，但我喜欢。没想到那个女郎冲进电梯，猛打小伙子，嘴里还骂到：你这个臭流氓！直到晚上睡觉之前，小伙子才醒悟过来，原来那个女郎当时说的是: Going down吧？And here's a rough and dirty translation:
There once was a [Chinese] guy who went to a foreign [i.e., non-Chinese] company for an interview, after which he got on an elevator to leave the building. When the door opened on one of the floors on the way down, the guy saw a sexily dressed woman, learning her body against the doorway of the elevator. In a seductive voice, she asked, "Loose enough, aren't I?" [In Chinese, this sounds a lot like the English "Going down?". More below.] The guy jokingly replied, "Yeah, you're pretty loose, but I don't mind at all." Before he knew it, the woman jumped in the elevator and slapped him, yelling "You damn pervert!"OK, so it doesn't work quite as well in translation, but let me explain.
That night, as the guy was laying down to go to sleep, he finally realized what the girl was trying to say: "Going down, right?"
Chinese speakers who use a lot of English will often throw in English words with Chinese, including the word ba 吧. Ba can have various meanings, but one common translation would be "right?" or "isn't it?".
Go into any Western law firm's office in China and you'll find them using terms like "M&A", "capitalization", etc., and even words like "contract" mixed in with their Chinese. There are perfectly good Chinese words for all of these, and yet they throw these in there anyway. The same is true in other foreign companies in China; English is frequently used, and English words will be mixed into the Chinese. That's why the joke specifies that he interviewed at a foreign company.
Thus, you could very well hear someone say, "Going down ba?" That's what our joke's sexy lady was saying. What our less-worldly guy heard was, "Gòu yíndàng ba?", or "Loose enough, aren't I?" "Gòu yíndàng" of course sounds a heckuva lot like "Going down", followed by the ba mentioned above. And therein lies the joke.