I was writing a response to a comment
's and it got out of hand, so it's getting its own post.
At the core, 95% of shounen manga are fantasies of power. Most of them specifically work with the idea of an "inner strength" that's greater than outer strength - because, you know, most people reading manga are not big huge muscle-dudes. So the idea that this person of apparently ordinary physical size and strength could become the hero that everyone looks up to and fears, because of some internal quality, is very appealing.
Manga depending on this idea - and this occurs in shoujo as well as shounen - usually have some visible symbol of "inner strength." Bankai and transformations are Bleach's. Displaying and using your inner strength is simultaneously an act of aggression and a form of communication - if two shounen manga characters beat each other down with all their inner strength, they nearly always at least respect each other at the end. Sometimes they become BFFs! (See: like, half of Ichigo's current relationships with guys. He has beat most
of these people up!)
In the most compelling shounen manga battles, when you agree to "show someone your true power," what you're doing is offering them a kind of intimacy, because you're showing them your soul.* And like in real life, making that offer requires a certain surety of oneself. This is why, when a Bleach character has summoned his Bankai for the first time in the past, he's always looked cocky or solemn or angry. He may also look tired or like he's under strain (Ikkaku
did, and I think Hitsugaya?), but he never looks scared, because it goes against genre logic for a person who's not sure of himself to voluntarily put his soul out in front of someone else. It would be self-destructive - hence the trope where someone who's panicking or deluding themselves about something tries to use their secret technique, and either can't summon it, or loses control and kills themself with it.
Now, in later
uses of the "true power," it's okay to portray it merely as a weapon, because the reader's already familiar with the character's soul, even if the opponent isn't. If every opponent reacts the same way to the revelation of the hero's power, the readers will start rolling their eyes. This is part of why characters like Ichigo just keep powering up throughout a shounen series. It's not purely escalation of the danger level; it's also a reassurance to the readers that the characters still have depths yet to be explored, and that they're growing and changing.**
(Some of the most successful manga of this kind are, I think, the coming-of-age stories, where these transformations mark a character's movement towards self-knowledge and responsibility. In my experience, shoujo manga does this better than shounen - Sugar Sugar Rune and Divine Melody are my favorite examples.
Though Divine Melody still has time to kill everybody and break my heart.
So, what all this was leading up to was: Soi Fong's first use of her Bankai, which is also the first time a female character has used Bankai in combat. ( Spoilers up to the end of Soi Fong's fight with Barragan (I don't remember the chapter number). )