Wolfbagging.
1. Tie a little bit of bacon (streaky is best) to the end of a piece of string (a least 2 feet long)
2. Have your partner swallow the bacon (while you maintain a good grip on the other end of the string)
3. Shag your partner up the arse
4. When the vinegar stroke is nigh, pull firmly on the string. This will cause your partner to vomit, which in turn will cause his anus and rectum to tighten giving you a lovely squeeze at just the right moment
5. Apologise to your partner and clean the mess up.
It probably works on girls if you can find one dirty enough (if you do - pm me with her phone number, I know some other games)