(This is today's bit of advice from the book Safety Toolbox.)
This is a variation of the hidden ball safety. A wall of balls provides overlapping shadow zones that together make a huge table area to set up a hidden ball safety. This enlarged table area makes it easier to roll a ball into it. Your opponent is forced to kick the cue ball off other rails in an attempt to make a legal hit.
The wall of balls is most often used to hide the cue ball. The cue ball can be more easily controlled and the larger target area allows for less precise speed control.
It can also be used to hide the object ball. In 9 Ball, when playing a defensive shot, shooting the lowest numbered ball behind a wall of balls makes sense. The size of the shadow zone means that you can be successful even when using one of more rails to get there.
During the early-game, it is easy to find multiple opportunities for using the shadow zone created with multiple balls. As more balls get pocketed, opportunities for even a double ball wall depends on table layout opportunities.
For experienced players, a wall of balls provides flexibility. Nudge the hidden ball up against any ball in the wall to reduce the kicking options. The more control you have over the ball being hidden, the tougher you can make it for your opponent.
A wall that extends out from a cushion is easier to move a ball into. If the object ball is hidden, the wall limits the various paths that the cue ball can use. If the cue ball is hidden, it limits the paths for kicking.
Another helpful hint – in the process of putting a ball behind a wall, the more distance you separate the cue ball and target object ball the better. The target ball gets smaller the further away it is. You can also use the wall balls to absorb the last little bit of roll on a ball you are positioning.
How you handle a wall of balls opportunity depends on your ability to manage your cue ball speed and spin. It helps if you trace out the paths of both the object ball and cue ball for the shot. This makes the target area easier to visualize.