By making the 'inner bow' thicker than normal. it explains it all in the TBB.
If your outer limbs are stiffer then you are concentrating the bend closer to the handle.
By doing this the tips return path is longer AND the inner limb is thicker. It is thicker because it is being acted upon by greater 'leverage' and thicker limbs have a natural faster return speed than a thinner limb. Of course set trumps all
A gain somewhere means a loss somewhere else normally....