Although I've read the other posts that say that heat is the only thing needed, I subscribe to the camp that says moisture is also needed to get wood to bend and keep its bend.
The moisture softens the wood grain and allows each long grain to slip relative to the grain next to it.
(Imagine a bunch of flexible pieces of straw forming a 1 1/2 inch bundle. The ends of the straw are all cut the same length when they are forming a straight bundle.
Now, if one bends the bundle of straw, the ones on the inside of the bend won't move much but those on the outside of the bend will slip resulting in the ends being shorter than the inside straw.
This happens because the length of the larger arc is longer.)
As for using just heat, I still remember one of our members using electric heat lamps to heat the wrist of his TC Hawken.
Without going into details, he charred the wood so badly he ended up throwing the stock away.
I don't think his stock ever did actually bend.