The Swinging Festival used to take place every year but was discontinued after too many people died. The following contemporary account was written in the late 19th century:
The Swinging Festival usually occurs at about Christmas time. Processions bear the ‘mock king,’ who is an official of high rank, to the big swing. Brahmin priests lead him to a carpeted platform where he sits with one foot placed on his knee to observe the unfolding events. He is attended by four Brahmin priests, two on his right and two on his left, until the three rounds of swinging have ended. This can take about two hours. If he touches the floor with his raised foot before the games are ended, the Brahmins were allowed to strip him of his property and clothes and chase him through the streets. Nowadays, he has to pay a hefty fine to the Brahmins instead.
Four or five men, who are dressed in white with tall conical hats, mount the swing and urge it towards a bamboo pole on which a bag of money is hung. The nearest man on the swing tries to grab it with his teeth. This is somewhat difficult and dangerous as the swing supports are 75 feet high. The first set of swingers who succeed get twelve ticals, the second eight, and the third four. After the third set has proved successful the Brahmin priests then sprinkle water as a blessing, and everyone returns to the palace.
The origins of the swinging ceremony comes from the scriptures. Concerned about the end of the world, Uma Devi contrived a bet with Shiva. A serpent was suspended between Putsa trees on the river, swining back and forth between them. Shiva stood in its path on one leg with the other crossed. If the serpent struck Shiva and he fell, that would signify that the world would end. But Shiva did not fall, proving that the whole of creation was secured and strong. The Swinging Ceremony compares the swing to the Putsa trees, while the space between its posts is the river.
Sources: “The Must See Sites in Bangkok” (Bangkok Metropolitan Tourist Bureau), “The 1904 Traveller’s Guide to Bangkok and Siam” (White Lotus) and “The 1894 Directory for Bangkok and Siam” (White Lotus)