I have to respectfully disagree and reinforce some of the comments posted on this subject:
1). Toasting or Charring the wood tempers the wood to make it slightly harder / stiffer. It is not just a color change. However, for maximum hardness benefit, the heat must penetrate into the core of the wood and not just on the surface even though just a "surface treatment will temper the surface of the wood. In other words, the tempering/hardness change penetrates only as far as the key temperature does.
2). Tempering or hardening of the wood helps compression weak woods. It does not aid in tension, in fact it can hurt tension weak woods. This is why it is not usually recommended for backs, just bellys however, certain woods (bamboo in particular) can handle and is quite condusive to tempering and this is why some successfully temper a bamboo backed bow.
3). Heat treating a piece of wood for hardness and heat treating for shaping are two different things. They should not be confused with one another. There are other posts on the differences here and elsewhere.
I have to admit though that I am quite a rookie bowyer yet, but I have been heat-treating wood for many years for other reasons such as fly rod-building, ship making, etc.