If you ejaculate, then you let the cum out.
What you are describing is edging, where you get to the point of cumming, but you hold off on releasing. I have never heard about the proteins nourishing, that sounds like BS. But if you jerk off till the point of cumming, but don't release, eventually you will see some gains. But you have to do this for quite a while, it'll take some time of continuing to do that before you see some gains.
If you edge for a while, but do release, you are working your PC muscles. What that will do is give you harder erections and allow you to control your orgasms, so you will be able to last a lot longer during sex. And if you like to be a showman, it'll help you shoot farther.
Now, I'm not as knowledgeable as others on here, but this is what I have interpreted. It all has to do with working your PC muscles. work on them enough, and you will see some gains and harder erections.