Complex sentence: The concave recess trapped gun exhaust gases, creating a pressure buildup in the recess which pushing the aircraft's nose to the left, cancelling the recoil of the large cannon, which otherwise pushed the nose to the right.
Split into simpler sentences: The concave recess meant that when the gun was fired, exhaust gases would create pressure on the recess and push the aircraft's nose to the left. This innovative feature cancelled out the recoil of the large cannon, which tended to push the nose to the right.