I still can't wrap my mind around this trapping concept....in my mind it's only creating a crowned back??? I have read how it is suppose to work, but I am not sold on it. 
VMB
Not sure if this is actually what is going on. But here is the way I wrap my mind around it.
When you draw a bow you feel a single resistance force. That force is made up of two reaction forces from the bow. The belly resists in compression and the back resists in tension, and they sum to the total resistance force.
The force you apply to a bow is evenly distributed throughout the back and the belly. But if a wood is tension strong, you need to apply a large force to overcome its tension resistance. Since the load you apply is balanced on the back and the belly, you need to apply that same high force on the belly. By doing this you are blasting right through its compression limits and causing set.
By trapping the back, you are reducing the force you need to apply to overcome the wood's tension resistance, bringing it closer to the wood's compression threshold. So now when you draw the bow, the compression and tension resistances are balanced and you are not overstressing one or the other.
