| Quote #1
"It is this possession, and not any employment of the letter, which bestows the power. With the employment the power departs." (33)
You can't eat your letter and have it, too. In other words, if you use the information in the letter, then you've blown all your power.
| Quote #2
"No? Well; the disclosure of the document to a third person, who shall be nameless, would bring in question the honour of a personage of most exalted station; and this fact gives the holder of the document an ascendency over the illustrious personage whose honour and peace are so jeopardised." (26)
Well, looks like dumb ol' G— is just as tricky a talker as Dupin. What he's saying is that whoever holds the letter has power over the royal lady. "An ascendancy" is the state that exists when one person or group has power over another.
| Quote #3
"But this ascendency," I interposed, "would depend upon the robber's knowledge of the loser's knowledge of the robber. Who would dare—." (27)
In other words, the only way the letter could be used to dominate the royal lady is if the royal lady knows she was robbed, and who robbed her. Yep. That's pretty much how blackmail works.