I can chime in here with experience. Me and my wife started trying this. I have done tons of research as well as bought books and dvd's.
All women can do this. It is the female prostate that makes this possible. A woman must first be aroused before trying to stimulate the g spot. Once the female is aroused the g spot can be stimulated by appluing pressure to it. Once the g spot is stimulated you can move to clitoral stimulation until orgasm is reached.
When the g spot is being stimulated the woman will feel the urgeto urinate. Most women will think they need to urinate and hold back. The woman should urinate and cmpletely empty her bladder before trying to ejaculate. When they reach orgasm they have to push like they ate trying to pee. They will expel a clear fluid that is odorless. It is prostate fluid much like a mans seminal fluid. Th part that is thin and watery. My wife likes it very much. She says its more intense than a clitoral orgasm.