Slapping downward on the lockdown bar slightly reduces ball's counteractions when you're trying to get it settled on the flippers. Or when the ball grazes the sling or other continuous surface, and you would like the sling to not trigger. And one assumes this adds less energy to the bob's swing than an opposing side-to-side or to-and-fro movement.

Of top-ranked players, I notice Zach Sharpe do this the most. With pinball and all other things, monkey see, monkey do.