Heat it in an oven , and then clamp it between flat boards . Soak it wrap it in paper or cloth and clamp it between flat boards .
The first could work but heating exposed wood results in rapid changes and possible cracking . The second method I never had much luck with .
Soaking ABW or cocobolo good luck .
But what I do , have used on Indian Brasilian ABW cocobolo and others , potato-chipped wood , and it does work :
You get two plates of mild steel 1 cm thick . As big as you need and no bigger .
Heat them up to 100 degrees C . Use a gas stove or any heater that will do it .
( I just heat them up till water sizzles on them , and as long as wood does not smoke and burn , but you can get a little bi-metal surface temperature guage cheap and it is good to keep records of what works best . It is better to use the lowest temperature that will be effective )
I use visegrips to handle the metal . It is heavy and care should be observed .
Put the offending back piece between the heated plates of steel and leave there till they cool down and the back will come out flat as can be .
One drawback :
Ever had a nice dark set and when you plane it down it is much lighter inside ?
Without discussing oxidation I have observed and other luthiers experts have confirmed same thing to me , that heating seems to draw the color to the surface .
So this fact should be taken into account when working up the wood .