The fight in the ballroom
From the early days of the Mistborn books, I'd been planning how an Allomantic gunfight would go down. I felt it the next evolution in what has been stylistically a big part of these books.
There is a fine line to walk in a lot of these sequences. I've made something of a name for myself in the fantasy world by attempting to mix some scientific reasoning with my magic systems. At the same time, Allomancy was designed precisely with action sequences in mind. I wanted them to be powerful and cinematic—and a cinematic fight sequence is often at odds with realism. (Watch two people who really know what they're doing fight with swords sometime, then watch any fight sequence in a film. Most of the time, the film sequences stray far from what would really happen.)
So, as I said, I walk a line. Sometimes, there are things I just can't do because they violate what I've set up as the rules of the world. Other times, I design the setting and nature of the fight specifically to allow for certain types of cinematic sequences. One thing I like a lot about Wax’s abilities is the power he has to manipulate his weight. There's some realism to what he does—for example, increasing his weight doesn't make him fall more quickly, but it allows him to do some powerful things while falling. Destroying the chandeliers is an example.
At the same time, I acknowledge that the weight manipulation aspect of Feruchemy is one of its more baffling powers, scientifically. Is he changing his mass? If so, he should become more dense, which I don't actually make the case when it plays out in fights. (Otherwise, increasing his weight enough would make him impervious to bullets.) So, if it's not mass manipulation, is it gravity manipulation, like Szeth and Kaladin do? Well, again, not really—as when his weight increases, his strength and ability to uphold that weight increase as well. Beyond that, Wax can't make himself so light that he has no weight at all.
So . . . well, at this point, the ability to explain it scientifically breaks down. I do like what it does, but I have to set its boundaries and stick to them—and accept that some of what's going on is irrational. (And don't get me started on what should really be happening scientifically when Wayne speeds up time.)