The image on the left below shows backlash in the y plane. Notice the groves on the ends of the part. These are supposed to have 90 degree corners, but instead, have odd, uneven shapes. The y plane belt is loose and as the y motor stops, the bed continues to travel, until the belt is no longer slack, overshooting the intended position. The bed then bounces back and forth until the belt tension is equal on both sides of the motor pulley.
The image to the right below shows how the part is intended to look. I tensioned the y belt just enough so that the belt had no sag along its entire length. You have to be careful not to over tension the belt which can cause the bearings to bind on direction changes, excess friction on the y axis motor, and the possibility of missing step counts.