The algorithm and/or references are often in the documentation (in this case, here). But I don't see them there.
If you type
type pcdownsample
you will see a listing of the file. References are often at the top of the file (which you might also able to see with just "help downsample"), and sometimes further down in the midst of the calculation.
I don't have the Computer Vision System Toolbox, so I can't check directly.
