The "Algernon-Gordon effect" is a term used by Charlie in his most advanced state of intelligence. The term refers to the mouse Algernon and himself who were used as guinea pigs in an experiment led by professor Nemur and Doctor Strauss to determine whether intelligence could be artificially enhanced. Charlie discovered that there was a direct correlation between the degree by which a subject's intelligence had been increased and the time it took for that intelligence to dissipate - the "Algernon-Gordon effect."
Charlie determined that:
"Artificially-induced intelligence deteriorates at a rate of time directly proportional to the quantity of the increase."
This means that the more intelligence you gained, the faster it would disappear.
The intelligence gained could not be sustained and the subject would regress to its/his original state. Charlie discovers through his studies that that was exactly what had happened to Algernon and that there is a flaw in Professor Nemur's hypothesis. Charlie is certain that he will regress to his previous condition. In this regard then, the experiment can be deemed a failure.
There are no clear references to which techniques were specifically employed to enhance Charlie's or Agernon's IQ. Only operations or injections are mentioned. One can assume that the procedures were performed on certain parts of the subjects' brains, but since no detail is provided, one cannot be entirely sure.
The operation that Charlie receives does not actually involve donating any brain cells from the mouse Algernon. In fact, the nature of the operation is purposefully left vague.
Keep in mind that the original story was written in 1958, at a time when medical science was significantly less advanced than it is now. Like many science fiction stories, the "science" is often left up to the imagination. All we know as readers is that Algernon's intelligence has increased dramatically as a result of the surgery, and we are asked to accept that fact on its face without any sort of explanation.
Fortunately, the story itself is so compelling that it really doesn't matter if we know exactly how the operation increases intelligence. The real focus, as in any good piece of literature, is on character development. When Charlie's intelligence increases, he does not become correspondingly happier. In fact, he struggles with the newfound knowledge that the world is a much more cynical and unkind place than he knew before, when he did not have the intelligence to see peoples' real motives and the effects of their actions.
The Algernon-Gordon Affect was a research paper that Charlie wrote based on the research he had performed while in his highly intelligent state. Artificially-induced intelligence deteriorates at a rate of time directly proportional to the quantity of the increase.'
He realizes that he had learned things very fast because of the experiment but he is also going to lose what he had learned rapidly. He tries to explain this to the doctor's but they are more concerned that they have success and do not understand his report. Later, after his IQ decreases again, he can not read or recognize the words in his report.
I could not locate any exact information on the operation but they are using some transference of brain cells from the mouse Algernon and placing them into Charlie's brain. It is the doctor's intent that Charlie will become smarter as the condition of his brain improves. However, the operation is with risks.