Don't do this with urine flow, it can cause bladder problems. If you're going to do it, you can do it just anytime (when not passing urine/semen). Do it in the car, at work, whatever. This make sense?

It makes sense its just not the same with no urine, does this help too?

You probably couldn't do it enough times in the amount of time you spend urinating to notice any difference. But once again, I would do it without urinating, if you're going to do it.

But does it help strengthen the pc muscles? To control premature ejaculation?

Yes aperently it does, so it's worth a try and must be just like training and working any other muslce. Whe I ejaculate, I use this muscle involutarily as like a reflex, it sortof pushes semen out. Is this like everyone else?

Everyone uses it to push semen out. And yes, this does strengthen the pc muscle, but believe me, only doing when you're urinating won't be enough to help (see kegels). As a word of caution, don't let it become to "strong." when men strengthen their pc muscles too much, they often have difficulty urinating.