Explain the "light as a feather stiff as a board" trick to me

I was browsing around James Randi’s site and came upon a reference to a levitation trick, described by Randi thusly:

I’d never heard of this before. It was asked about here a couple of months ago, and the asker was directed to do a websearch for “light as a feather stiff as a board”. I did so, and found loads and loads of instructions of how to do the trick, but none that explained how it worked (apart from one seriously goofball theory regarding biomagnetism, which claimed that the liftee had to be facing West for the trick to work). I understand that each person lifts only 25% of the person’s body weight, but that’s a lot to lift with one index finger…

Anyone care to explain?

The way we used to do it at university was to try and do it on someone, we’d all fail, and then put all our hands on the persons head, then we’d try it again, and the subject would shoot up into the air. Tres bizarre.

Randi explains:

"To conjurors, of course, the illusion known as "levitation’’ is well-known. It is accomplished by a number of means, some very technically complex, others quite simple. A popular trick performed by persons with pretensions of psychic powers, consists of asking four or five volunteers to place one of their index fingers beneath the armpits and knees of a seated person (or, in the case of a standing person, beneath the insteps, chin and elbows) and by simultaneously lifting upwards, cause the person to rise. The even distribution of the weight allows the trick to work easily. This was described by Samuel Pepys (1633-1703) in his famous Diary as being done by French schoolgirls upon a corpulent pastry chef. "

A little math makes it clearer. Lets say our seated subject weighs 200 pounds, including the weight of the chair. If you have four volunteers doing the lifting, then each one must lift 200 / 4 = 50 pounds, and since each one is using both index fingers, each one must only lift 50 / 2 = 25 pounds per finger. Easily done.