Backing the ball is a concept that works best in American rules (hence the example) but can be used in the Association game and 9-wicket quite often. The concept is to discourage an opponent's shot at a ball on court. In this example the striker (Blue) has an opportunity to move the ball in question, but this play can work from a distance when moving the on court ball is not an option.
So Blue has just made hoop 2, but realizes that hitting partner will not give her a great (90% or better) chance of making hoop 3. With partner on court and no spent ball in the game (yellow is still for hoop 1) setting up partner is too risky. In fact, Red is so close and has Black at its hoop that he will assuredly try to hit that ball. Blue's best option is to go out of bounds, but there's only one place that will truly discourage Red's shot. Anticipating Red's line of play (Red dotted line), Blue shoots out of bounds behind the Black ball (Blue Line). This adds a huge risk to Red's supposed shot and encourages Red to take a less aggressive shot, like cornering.
Whew! Crisis averted.
Note: This will not work when Red has another ball to hit (i.e. Yellow).