Well he's not holding onto the wood so it's staying there by some other means. Looking at the piece of string just visible between his legs and the metal (bolt?) end in the centre of the wood I think it is held there suspended by cord/chain from the centre of his wrist restraints and perhaps tied or fixed to his balls or even round his thighs. His balls may also be tied under him to his tits to prevent any other position being possible. There is a covering to the ass side of that board so given your devious plans it's not there for show but to be another predicament. Or maybe just to prevent splinters:)
The chain and magnet I agee with jimmycontour.