In Stave 1 why does marley jaw drop when the bandage was removed?
Scrooge has already told Marley that he doesn't really believe his senses because "'A slight disorder of the stomach makes them cheats.'" In other words, Scrooge thinks that Marley is just a trick of his imagination, a minor hallucination caused by having eaten something either spoiled or not fully cooked. Scrooge even makes a joke at Marley's expense when he says, "'There's more of gravy than of grave about you, whatever you are!'" He employs a pun, a play on the words gravy and grave because they sound so much alike, and Scrooge means that Marley's appearance has more to do with something he ate than it does with death. When Scrooge finally calls Marley a "'humbug!'" Marley loses all patience.
At this the spirit raised a frightful cry, and shook its chain with such a dismal and appalling noise, that Scrooge held on tight to his chair, to save himself from falling in a swoon. But how much greater was his horror, when the phantom taking off the bandage round its head, as if it were too warm to wear in-doors, its lower jaw dropped down upon its breast!
Marley unties the bandage around his head, a wrapper that is meant to keep his jaw closed in death, in order to terrify Scrooge into a belief in him. If Scrooge refuses to believe in Marley, then the ghost(s) will have no effect upon his chances for salvation. Marley's jaw falls to his chest because he has been dead for some time and so, theoretically, the muscles that would allow him to be able to hold his jaw closed would have lost their integrity. In order to convince Scrooge of his reality, Marley allows himself to appear frighteningly dead, a real corpse, in this moment, so as to impress upon his old friend the seriousness of the situation.
All very nasty! Traditionally, when corpses were prepared for the grave, coins were placed on the closed eyelids and a bandage placed round the chin to keep the eyes and jaw closed during the process of rigor mortis - literally, the stiffening of the tissues that occurs during the first few hours after death (in humans, typically after about 2 hours, and which gradually passes off over the next 72, beginning with the head, and passing off in the same order, head first.) The stiffening causes the jaw to gape open - hence the bandage. Once the stiffness has passed, the corpse becomes floppy, and the closed jaw would 'drop', especially if - as in the case of Marley - the corpse were standing upright.
Part of the horror-factor in Dickens's story is Marley's Ghost presented as a corpse: in his final revelation to Scrooge, Marley is dressed for the grave as well as weighed down by his chains and strong-boxes, human (and dead) as well as fantastical. Dickens works a double horror, on Scrooge, and on the reader: the macabre image of the corpse releasing the bandage and its dropping jaw is the moment when Scrooge believes in the Ghost ("I do, I must...") - until now he has scoffed at its existence. And there is surely the inevitable mirroring of the Ghost's literal jaw-dropping with Scrooge's 'jaw-dropping' horror with ours - a horror worked on the reader, for we 'gape' when we witness shocking spectacles...this scene was intended to shock, and it does.