You really have to hear the shiur to understand the context. A VERY simplified explanation is that when you do things you shouldn’t be doing (or that are assur, don’t remember exactly), any benefit you get is fed to you from the Soton. If you run a prostitution ring, the money you make wasn’t designated to you on Rosh Hashana, it came from the sitra Achra to encourage you to continue, so anything you do with that money is going to be tainted and you can’t get schar from it.
That is my SIMPLE version of what he said, he explains himself and sources.