I know how to do it, but never done it myself.
Saw it done at a formal dinner once - the lady on the next table started to choke, and the guy four seats down on the other side calmly came round, dragged her chair back, applied the manoevre, and she spat out a large wodge of food, and went from bright red back to her normal colour.
We all thought how brave he was to perform this on a complete stranger, but it turned out it was his wife, and they were at a business dinner where everyone has to sit away from their guest!