Hi Sidney, very good question that does deserve some form of explaination (by the way it is now known as impeding, poncy I know but hey!). When shielding the ball going out for a goal kick (or corner) providing that the ball is still within playing distance this is a perfectly legitimate tactic (attackers do it when going into the corner for example). However, if the ball is not within playing distance than a free kick should be awarded. Usually, they are given in open play when the player is impeded with little or no attempt to play the ball. The goal kick type scenario has crept in but on occasion I do give against the defender if he deviates from the ball ie jinx about or sticks his backside out.
Hope that helps (I've read it back and it is still as clear as mud!)