This is basically a piece of code to draw a circle window with a given radius. Unlike the one that comes with Effect Tool, this one generates the circle on the run, not using a pre-computed table, allowing you to change the radius anytime you want.
The ASM file comes with some customizable options, such as whether the circle is centered at a fixed position or at Mario, the initial radius and some other stuff.
It's only compatible with SA-1 though.
Notes: don't use it with message boxes, and make sure your level end isn't in the same level of this effect, otherwise stuff will look glitched up when you cross the goal tape or use the keyhole.
The only thing I would say is that it really should be a library, with the defines for the various options somehow abstracted out, so that the user can just call the code with different options for different levels, instead of inserting the same code mutiple times.