How again was it

Could anyone please explain the part in bold to me. I didn't get it.

▪"𝗛𝗼𝘄 𝗮𝗴𝗮𝗶𝗻 𝘄𝗮𝘀 𝗶𝘁 that you convinced me to do this?" the boy said to his friend before they jumped out of the plane on their first sky dive.

The first step to understanding the role of again here is to realise that it does not modify any particular part of this sentence, but rather the sentence as a whole. Its syntactic role would probably be clearer if it were separated by commas from the rest of the sentence.

What the word conveys is that one is aware that the matter has been discussed before. Why would one raise the matter again, if one is aware of its having been discussed before? It could be, as LPH has explained, that one has forgotten what was said then, and is asking that one's memory be refreshed. If the sentence is taken at face value, that would be the explanation.

The questions of this kind are, however, often used as rhetorical devices, and not intended to be taken literally. Chances are that the boy in the OP's example is not literally asking to be reminded of what was said in some previous conversation. After all, if they are both about to jump, there is no time for that. The boy hasn't literally forgotten the reasons that led him to agree to try sky diving. It is rather that now, that the jump is imminent, the reasons don't seem as convincing as they did before. The sentence was probably intended as a figure of speech, expressing his present fear and hesitation.

One could easily imagine other scenarios in which the questions with again would be used in the same way: 'Why, again, are we going on this hike?' (asked as one is waking up, and staying in bed seems much more appealing), 'Why, again, am I supposed to go on a diet?' (asked as one is eyeing a large slice of chocolate cake).

The boy is remembering that a certain reasoning held to him by the person he is talking to was what convinced him to agree to try a sky dive;it happens thouh that he doesn't remember the exact reasons that were given to him; by saying "again" he is expressing the wish to be recalled of that or to recall it himself and he'll probably try to recollect.

However, I think this turn might be rather colloquial; it doesn't appear in the books (Google Books). Neither does "how was it again", which sounds better anyway.

You don't find either those two: "how is it again,how again is it" (ngram).; nevertheless, if "how again is it" is not used you can sometimes hear "how is it again".

"What was that again, what is that again" are however standard English (ngram). "Again" is explained in those phrases as in the other ones.