It is said that in order to release Final Getsuga Tenshou, Ichigo has to be Getsuga himself. But, what does it mean? How this Getsuga mode worked? Is it only Zangetsu exclusive? Is it cancelable and be reused?
To understand how Final Getsuga Tenshou (FGT) works, we need first to understand what happened with Ichigo when he wanted to learn it.
Ichigo and Isshin was in the Dangai (Precipice World) a tunnel that connects Living World and Soul Society. Isshin stopped Ichigo and noted that the train was not there and that while it usually is a bad thing, it was a good thing for them since because the flow of time is different in the Dangai than that in Real World or Soul Society, it is a good time for him to teach Ichigo the FGT. Isshin then told Ichigo that he can only attain the technique by asking his Zanpakutou to teach it to him. He then told Ichigo to sit crossing his legs and start communicating with his Zanpakutou, which is similar to what meditation is.
What happened with Ichigo when he entered his inner world is that he found that all the tall buildings are under water. He was surprised but then Tensa Zangetsu shows up and tell him that he can breathe just fine even under water since its just his inner world. Ichigo asked Tensa Zangetsu to teach him, but Tensa Zangetsu refused. Hollow Ichigo shows up and then fused with Tensa Zangetsu and then fight Ichigo, claiming that if he persist on trying to learn the technique, Hollow Tensa Zangetsu will kill him. They fought, but Ichigo keeps getting beaten.
Then Ichigo realized that there is something wrong with the fight. Hollow Tensa Zangetsu could have killed him from the first strike, yet he didn't do it. He then understands how he can learn the technique, that is by accepting Hollow Tensa Zangetsu's blade. Thus he let the blade pierce him. Hollow Tensa Zangetsu then told him that that is the right answer for Hollow Tensa Zangetsu is Ichigo himself. Ichigo then proceeds to fight Aizen using his newfound power.
Now what does all this means? This means that to learn (to use) Final Getsuga Tenshou, Ichigo needs to understand himself and accept it as is. That's why it was said that to use Final Getsuga Tenshou, he needs to be Getsuga itself.
How does it works? If we see how he attained it, he probably need to enter a special state of mind, which he can reach by meditation. In Zen teachings, there are some special states of mind, namely, Shoshin, Fudoushin, Mushin, and Zanshin. Mushin in sports is often called as "Zone". If you watched Kuroko no Basuke (The Basketball that Kuroko Plays) you will see that when in the "Zone", all abilities goes up drastically, which in Ichigo's case would be his reiatsu goes up so high that even Aizen no longer able to comprehend it.
Is it Zangetsu exclusive? Maybe. That question cannot be answered with exact answer. The only thing that we know is that Isshin knows about the technique, and quite possibly has used it before, and that Ichigo was able to learn it as well. If we go by how Ichigo attained it, then it is very possible that others might be able to learn similar technique although it won't be called Final Getsuga Tenshou since they only ones who has been shown to be able to use Getsuga Tenshou are Isshin and Ichigo.
Is it cancel-able and reusable? If we go by what Kise Ryouta did during the match between Kaijo and Seirin in the semi-final of the Winter Competition, then yes, it can be canceled, assuming that FGT works by entering a "Zone", and that it works the same way in Bleach universe. What we know for sure is that Isshin/Tensa Zangetsu (I forgot which one) said that once he used the technique he will lost all his powers. He will turn into a normal person. This means that it is not reusable.
Unfortunately, There are no solid answers yet, just theories. As it currently stands, it seems unlikely that we will ever find out either, as the Manga is only a few chapters from ending, Though it may make a return to defeat the final villain and we may get to learn something about it then.
Theory 1) Ichigo fused with his zanpaktou sword spirit. This one does not make much sense, since the zanpaktou is already a part of its wielders spirit, but his sword did disappear, so it has some merits.
Theory 2) It was some kind of Shinigami, Quincy, Hollow, or unique technique to the Kurosaki Family (specifically on his fathers side), a lot like what Uryu did when he fought Miyuri, since both lost their powers after the fact. Though there are several big differences depending on where you place it, we only know that Isshin could supposedly do it as well, but cross species abilities are not explored much outside of Vizards/Arrancars.
Theory 3) The most likely theory as it stands, is simply that Kubo wrote himself into a corner. He made Aizen too strong, So he had to come up with a way to make Ichigo Stronger.
Theory 4) This one is the newest, but also spoils a lot from the last Arc in the manga, Some very big plot points are revealed about Ichigo spoiled here:
Ichigo is in some way partially all 4 of the main races, and as we find out, His quincy part suppressed his Shinigami part, and then somehow used some of its own power and the Hollows power to give him his regular powers that we all knew and loved. Eventually, Ichigo is able to fully access all of his powers, and we see his new regular strength is well beyond Yamamoto's and Aizen's, with his reatsu being visible to the naked eye in just his shikai form. This theory is simply that Ichigo used a technique that forced any limiters to be removed, and accessed his absolute power. Like Uryu again, The removal of the limiters caused damage to his Spiritual power network, and he could no longer produce more Reatsu, so he slowly drained it all away until he was human again.
Lets hope in the next few chapters we find something out, but as it stands, If we don't, we probably never will.