A major function of the P.C. muscle
When you contract your p.c. muscle you can feel an increase in the size of your penis. The explanation for this is that the veins leaving the penis pass BENEATH the pubic ramus ( through the holes in the pubic bones), but OVER a part of the p.c. muscle.(so that the muscle is like a sling under the vein).Thus , when the p.c. muscle is contracted,the veins are clamped against the pubic bone thereby stopping venous flow from the penis and making the penis enlarge because of venous engorgement.
The longer you can contract the p.c. muscle the longer you can keep the blood in the penis =s a larger, stiffer ,penis.
The P.C. muscle is one of the muscles that is not totally voluntary muscle, but only partly so.That is, you cannot control it all the time with your mind— in many instances it is involuntary, for example after an orgasm it contracts and relaxes on it own and you cannot control it. Therefore you can contract it just so long and then you lose the ablity to hold it any longer.