When the belt moves over the pulley to transmit power, the tension in the tight side is more than the tension in slack side. As the belt material is elastic, it elongates more on tight side than on slack side, resulting in unequal stretching on two sides of the drive. This motion of belt relative to driving and driven pulley due to unequal stretching of the two sides of the drive is known as creep. The effect of creep back on the driving pulley is to slow down the speed of the belt with respect to driving pulley and the effect of creep forward on the driven pulley is to slow down the speed of driven pulley.