You can't just do it on demand, it has to be coaxed out of you by someone who has some idea (at least) of what they are doing. I didn't believe that I could do it until I was introduced to this gentleman at a swinging club and he got me squirting within literally minutes of being let loose with me. To say I was in shock was an understatement.
For anyone who would like to try it there are two ways that I am aware of, but of course all women are different, so it might be slightly different for other ladies.
1. Using a kind of royal wave over the g-spot, quite firm rapid strokes.
2. using your manhood (with condom on) rapidly run it while it is hard over the clit.
The second one has only happened to me once, but it was a client who did it to me and I was totally shocked, as I did not know it was possible to gush by doing that.
I've also gushed with a client who used a vibrator on me. He tilted it slightly upwards and rapidly moved it in and out while I used my magic wand on my clit.
Oh and sometimes it has happened during penetration while I am doing cowgirl.
Hmmm... I guess that would be 4 ways the, not 2