In physical pool, striking the bottom of a cue ball against the rail requires raising the back end of your cue stick (cue elevation). 

FlyOrDie prides itself on an advanced 3D simulator where "balls react just as they would on a real table." In real-world physics, a pool cushion is made of vulcanized rubber. 
When a cue ball rests against it, the rail isn't a solid brick wall, it deforms and compresses under force.
When you strike the ball downward, it pushes slightly into the rubber cushion, clearing a millimeter of extra space for the tip to impart backspin right before the ball springs forward off the rail.