I'll be honest, it took me awhile to realize this and having it have to take this long may ultimately make it poor writing but my realization had made me have a greater appreciation toward Ikari and I want to share my opinion on why Trunks achieving this form makes sense. I find Ikari to be the form that allows Trunks to use the third grade Super Saiyan state minus the disadvantages. I still see the comments saying how it was pulled out of his ass, a cheap power up, etc but it perfectly aligns with already established scenarios; this isn't the first time Trunks had the help of anger to transform, in Dragon Ball Z episode #162 Trunk transforms into the third grade state after watching Cell beat Vegeta down. The transformation is similar for both of these in that it makes Trunks lose his pupils.
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Not to mention that each time he transforms into Ikari his muscles become bigger and then compress themselves.
First time transforming
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Fight against Black & Zamasu
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Before using the Galick Gun
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Heck, him even putting his arms up like that is similar to this.
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I think this had more a lot to do with show and don't tell. Thoughts? Sorry if this was already discussed in such a way before, I didn't see anything when looking.