yeah eiri made a good point about the cirvical opening thing lol. it doesnt strengthen the cervix. i dont think this is possible.
pc exercises tone the muscles around the vagina and doing them can help prevent incontinence in later life.
because it is difficult to explain where the pc muscles are, people use the example "imagine you are trying to cut off the flow of urine while you are in the middle of peeing". these muscles you use are the pc muscles.
however you should not do the exercises while you are actually peeing. i'm not sure exactly why, but i have been told by a professional not to do it. you can do them anytime and anywhere, so i guess there's no reason to do them while peeing anyway.