In reality, the participants are tricking their minds, by way of the chanting, into believing that the person being lifted is "light as a feather". The body still reacts to the command from the brain, but the mind perceives it differently. Simply put, five (example) people can easily lift one person, especially when those five people are tricking their minds into thinking that the person is light-weight.
In one variation the person being lifted is told a story about their death and asked to imagine it happening to him or her. It serves the dual purpose of "freaking out" the participants and convinces the participants that it will be easier to lift this person.
In many versions, each of the (in the example) five people lifting the other person uses only one or two of his or her fingers on each hand to do the lifting. It is particularly easy to lift a heavy weight when it is evenly distributed amongst a group of four people. The phenomena of the weight seeming less on the second try around or after some sort of ritual is due to increased focus and the "lifters" being more in sync with one another.
Another reason for the apparent success of the levitation is the "self fulfilling prophecy" concept. The lifters know it's supposed to be heavy and not work the first time so subconsciously they don't put in the effort required to do the lift the first time around. The second time around they are focused and putting in the effort in order to prove to others (and even moreso themselves) that it actually works.
In an episode of South Park, "Marjorine", the game was played at a girl's slumber party. Butters, disguised as a girl, calls them all witches after he sees them chanting and easily lifting their friend.
In an episode of As Told by Ginger, Courtney and a few of her friends play this game with Ginger.
In an episode of Captain Planet, Sludger can be seen playing this game prior to a fight against Captain Planet.
In the film The Craft (1996), several teenage girls, who are involved in witchcraft, play the game and successfully manage to levitate their friend.
In an episode of The Girls Next Door, when Kendra's mother asks her what they are going to do at the slumber party, Kendra replies "Play light as a feather, stiff as a board." Later in the episode, the girls unsuccessfully play the game.
In episode 2.02 and 6.20 of Gilmore Girls, Lorelai mentions the game.
In an episode of Big Brother 9 the housemates unsuccesssfully try to lift Kat using this method.
In an episode of Even Stevens, the game is suggested at Ren's slumber party