My understanding is the “shielding” is special case of impeding, and is legal only when the ball is going to cross a touch or goal line. My questions: a) is shielding also legal anywhere on the field, to keep the opponent from playing the ball; b) can the shielding player use his rear-end, arms, or contort his body in ways not associated with a natural upright playing stance, to achieve the shielding?

Answer (October 20, 2012):
Your current understanding regarding impeding and shield is slightly flawed. Fortunately, we can fix that. Let’s start with a quote from this year’s Laws of the Game and go on from there.

From “Interpretation of the Laws of the Game and Guidelines for Referees” (back of the book):

Shielding the ball is permitted. A player who places himself between an opponent and the ball for tactical reasons has not committed an offense as long as the ball is kept within playing distance and the player does not hold off the opponent with his arms or body. If the ball is within playing distance, the player may be fairly charged by an opponent.

Shielding (a) is not “impeding” in any form and (b) it can occur at any spot on the field, not only when the ball is about to cross a boundary line.

If the player can legally play the ball and the ball is within playing distance, the player may shield as a tactic to prevent an opponent from getting to the ball (provided, of course, that the shielding does not involve holding). If the player cannot legally play the ball or if the ball is not within playing distance, such shielding becomes “impeding the progress of an opponent” and should be penalized by an indirect free kick.

In response to your query part b (unnatural positioning), one way of attempted cheating during shielding is “making oneself bigger,” the same sort of action used in some cases of deliberately handling the ball. An example of this would be a player shielding the ball and extending his arms straight from the shoulders or moving them around, an unnatural thing to do. No player shielding the ball from another is allowed to use his (or her) arms or any other part of his body for other than maintaining balance — which does not include pushing off or holding the opponent. If the player is simply maintaining balance — in the opinion of the referee — then an opponent who initiates contact with the player who has the ball is guilty of charging illegally. If the player with the ball is holding out his arms or a leg not to maintain balance but to obstruct the opponent, the player has committed an indirect free kick offense, provided no contact occurred. However, if the player with the ball initiates any contact, then he or she has charged, held, or pushed (all direct free kick fouls) and must be punished accordingly.