You practice assignments based on the premise the ball will be kicked to a certain corner or side of the field. Your lane or area assignment can also be based on this. You will also begin your sprint down field with the idea the ball will be kicked deep right for example. When the kicker kicks it to the middle or even left it screws all of this up and can lead to big returns. This is what Kelly is referring to. It absolutely pissed my college coaches off royally when the kicker would not execute the directional kick as planned.