The key to a sand shot is to make sure you take sand. It is not a sand shot if you pick the ball cleanly.
The sand literally propels the ball out of the bunker. For more roll, catch more sand. More sand equals less backspin. Handy for those longer bunker shots.
For more spin, meaning the ball checks up, hit just 1-2 inches behind the ball. To achieve this: You MUST keep your body quiet, maintain weight on your front side, and accelerate through the sand.