The emergency stop, quick stop, or panic stop, depending on what you want to call it, is one of the avoidance drills taught in the LAB traffic skills courses. It's a skill you need to learn and practice, because as you've apparently found out, when you need it, you need it badly ;-)
For all bikes, the braking power of a wheel is directly proportional to the amount of weight that the wheel is bearing. When you're just riding along, your weight is fairly evenly distributed between the front and rear, with maybe a little more to the rear wheel.
When you and your bike are traveling forward, due to your momentum you will slow down only if something pushes back at you from the front. The brakes cause the road surface to push back on the tires, and that push slows the bike down. Because you are up higher on your bike, and the push is down at the road surface level, while the push slows the bike, the rider’s momentum tends to keep him or her moving forward, shifting weight forward to the front wheel.
The stronger the braking force, the more weight is shifted forward. If too much braking force is applied, the weight on the rear wheel is lessened to the point that the wheel loses traction, and tries to come up instead of pushing down on the road, and since there is nothing to hold the rear wheel down, the back end of the bike comes up and the rider is launched over the handlebars.
What you want to do is to use both front and rear brakes, and keep the rear brake from losing grip and tossing you over the bars.
When you're just rolling along and need to do a panic stop, here's what you do:
Squeeze both the front and rear brake levers -- the front brake about two to three times harder than the rear. As the bike slows and your weight shifts forward, most of your braking power is going to be in that front wheel. At some point, you'll feel the back wheel start to skid, or even start to come up off the road, because it's no longer bearing much weight, and has lost traction. That's a signal to let up a little on the front brake until the back wheel no longer skids.
To help the matter. level your pedals. stand up, and thrust your butt back off the saddle and over the rear wheel to shift your weight back. This is where some experience mountain biking really helps, if for no other reason than mountain bikers find themselves needing this little trick more often than road riders. A good rider can put his belly button or his chest over the saddle and really shift that weight to the rear.
As soon as you slow down enough, unclip one foot and put it down when you stop. Depending on the situation, you might need to come to a full stop, or you may find it safer to use the panic stop to slow down significantly, and then do a quick turn to avoid whatever it was that caused you to brake in the first place.
It's a good skill to practice... get out in a vacant parking lot somewhere and practice coming to a stop as quickly (and safely) as you can from your usual cruising speed...
Tom