Back in Sister 2, it was said that Elliot needed to spend time transformed if he wanted to stop getting girly spells. Back in Indiana Elliot and The Temple of Swedish Furniture, Tedd was working with Elliot to try to learn more about magic and figure out how to speed up the process of getting non-girly spells and stopping the excess energy buildups.

What wasn't made clear in those comics was precisely why Elliot needed to spend time transformed (beyond it simply using up energy), or precisely why their first spells reflected those the diamond initially gave them so closely. I don't think anybody expected there to be a logic behind it beyond it being convenient for the plot, but there's at least a little more more logic to it than that, and it's based on how the magic system works as a whole.

Granted, Tedd may be partially or completely wrong here, but even if he is wrong, there is a rationale to it beyond "because I said so". Of course, this was a conclusion he reached in a fury of glowing brilliance, so really, what are the odds of him being wrong about this?

Wait, glowing brilliance? Since when could he... wait a minute. Also? Wait another minute... And also, this minute! Okay, that last minute was unrelated, but can you really blame me for linking to it?

...Okay, maybe YOU can, but most everyone else can't.