If you have a development environment, you can apply the patch there and just check that it behaves the way the author thinks it should, even if you don't understand how it works! Just leave a note to that effect if you review+ the patch. Don't worry, at least one other person will eyeball the code before it gets committed.

(If you don't have a development environment, you know you can get one really easily, right?)

There have been some patches with very good looking code that I have left alone just because I didn't know enough about the part of the site they were changing to test them with confidence.