Okay, I just saw the video over and over and here are my conclusions.
First, let's go to the ending of the video which fRobin starts with a wind tome, then uses Arcfire, and right before the atack there are flames that surround Robin. Then she uses Arcthunder, and again, right before the atack there are sparks that surround her.
In the middle of the video, when mRobin starts the sequence of Thunder, Elthunder, Arcthunder and Thoron with a thunder tome, there are no sparks before the atacks at all.
So I think, we can rapidly change between tomes, and I really think this is done by neutral B, and while every magic attacks we have seen so far are forward B, and the wind slices we have seen are actually down B. Plus I think, if we run out of tomes (if we stick with only one magic and don't change it) Robin will perform the animation which he throws away the tome, and it looks completely vulnerable and highly punishable.