Muggles' Guide to Harry Potter/Magic/Expelliarmus

Overview
The Expelliarmus spell is the Disarming spell; it knocks the victim's weapon (usually a wand) out of his/her hands. It can also be used to knock other items out of the target's hands, but we seldom see this use. When it is performed with more power, one can blast the opponent off their feet.

Extended Description
We see this spell first used by Professor Snape in the Dueling Club; Snape casts it on Professor Lockhart, disarming him and throwing him to the end of the platform on which they are demonstrating. Later in the same book, Harry casts the same spell on Draco Malfoy, causing the book that he is holding, Riddle's diary, to fly out of his hand. From this point on, it is used quite often when there is a duel between wizards; if you can get your opponent to lose his wand, you do have a great advantage over him. Later on, Draco Malfoy would use this spell on Dumbledore, unintentionally earning ownership of the Elder Wand.

In Harry Potter and the Deathly Hallows, Harry uses the spell against a Dark Eater during the chase, which causes them to recognize him. Because of this, Lupin warns Harry against the further use of this spell, suggesting that he use more powerful defensive magic.

It is, interestingly, the spell Harry uses to defeat Voldemort in the final book. Harry was, by this point, aware of his own mastery of the Elder Wand and was effectively immune to Voldemort's magic by this, but he needed to cast at least one spell, and it was the Disarming Charm which Harry chose to use. This contrasts with Lupin's earlier warning against using it too often.

Analysis
As mentioned, at one point Harry does use this spell to get someone to drop a book; as the diary is obviously not a weapon, we can see that this spell will act on anything that is held in the hand, not just a weapon or wand. It's possible that there is a mental part of this spell as well, envisioning the specific object that you want to have it act on; alternately, it could work on whatever you are pointing your wand at. In Harry Potter and the Prisoner of Azkaban, in the Shrieking Shack, Harry, Ron, and Hermione all cast this spell at the same time, and it not only pulls Snape's wand out of his hand, but also throws him against the wall. This leads us to believe that the power of the spell is additive when cast by multiple wizards on the same object.

Despite the fact that the charm can do things other than disarm, its main function is to get an attacker to drop his wand; as such, it is referred to as the Disarming spell. This is confirmed by the word's origin, "expelliar" meaning to "be expelled" and "armus" meaning "weapon".

Questions

 * 1) In the seventh book, it appears that this is taken to be Harry's "signature spell". Why would that be?
 * 2) In the seventh book, we learn that there can be other effects from the use of this spell. What would those be?