Back spin is to draw the cue ball backwards after hitting an object ball. It is done by hitting the bottom part of the cue ball; it can be hit at the center of the bottom, bottom right, or bottom left depending on how you want the cue ball to draw. The amount of power matters in how much you want the cue ball to travel while spinning backwards (hitting it with too little power will cause the cue ball to loose its spin as the friction from the cloth will transform the spin into a top spin). The height in which you hit the cue ball at determines how much spin will be put on the ball, this means the lower = more spin,

higher = less spin. However hitting it too low will cause the cue ball to jump (illegally)