From what I've read of the different methods, in the end its the heat that allows for the bending. Oil is used to transmit it.

In methods that have been described here the wrist was protected with foil to prevent the oil from affecting the finish, so it is not necessary for it to come in contact with the wood.

Its the heat.

JC

"...it is always advisable to perceive clearly our ignorance."ť Charles Darwin