The problem is that the guidelines do not explain how to do it. The precordial thump has really been de-emphasized when compared to the 2005 guidelines.
A strong precordial thump can generate from 5 to 30 Jules of electricity. The heel of the fist is swung down over the lower half of the sternum forcefully, from a height of 12 inches or so. Some advocates say that to get the amount of energy needed, two hands should be used, at a height of about 2 feet.
I have seen it approximately 5-6 more times, effective about half the time, and have performed it twice, with a 50 percent success rate of conversion of the rhythm. Research varies on overall percentages of success, but, at best, about 25 percent success rate overall.