I start making test cases until it breaks, then I rewrite it. If it doesn't break, by the time I have likely figured out how it works and I leave some comments

I start making test cases until it breaks, then I rewrite it. If it doesn't break, by the time I have likely figured out how it works and I leave some comments to make it easier for the next person to understand it. Most of the time though I just try to forget that I saw it. Once I've written a comment like "there be dragons" to warn others from threading deeper.