"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.
"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.
"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.
This functionary grasped it in a perfect agony of joy, opened it with a trembling hand, cast a rapid glance at its contents, and then, scrambling and struggling to the door, rushed at length unceremoniously from the room and from the house. (88)
When Dupin gives G— the letter, a transfer of power takes places. G— now has the power to claim his reward and win the royal lady's eternal gratitude; or he can use it to blackmail her, just like D— was doing. As soon as G— gives up the letter, he gives up the power.
"And the identification," I said, "of the reasoner's intellect with that of his opponent depends, if I understand you aright, upon the accuracy with which the opponent's intellect is admeasured." (97)
The narrator is reiterating Dupin's idea that, the more accurately you can measure another person's intelligence against your own, the more power you have over that person. This is interesting because it doesn't say that whoever is smarter has the most power. Nope, you can be way stupider and still win. So long as you know they are smarter, you can always anticipate their moves. (But doesn't that end up making you smarter?)
"I protracted my visit as long as possible, and while I maintained a most animated discussion with the Minister upon a topic which I knew well had never failed to interest and excite him" (116)
Hey, Shmoopers, want to win friends and influence people (and also skip reading an entire book telling you how to do that)? Talk about things that the other person is interested in.
"For eighteen months the Minister has had her in his power. She has now him in hers." (120)
D—'s power over the royal lady depended on her knowing he had the letter. Now, her power over him depends on his not knowing that she has the letter. Whoa.