1. Set fluid mode so it's possible to use wand underwater.
2. Click wand on something with the appropriate blocks in your inventory, despite having no wireframe that suggests that's possible (#27).
3. /wandOops
   There you go, now you have hollow air blocks where you had fluid blocks before.

To be clear, it's a bug.
Reasoning:
Especially with #27 hiding wireframes underwater, the path to accidentally using the wand there, needing /wandOops to fix it, and having /wandOops not actually restore the previous state is all too easy to hit.
Manipulating fluids is a big pain. Presumably this is why you think that quickly removing quite a bit of fluid could possibly be a feature. But that's a double-edged sword; having to manually replace all the fluid tiles this bug can destroy is just as big a pain.