Braking effectively on a mountain bike

 Why the front brake?

Using the front brake stops your mountain bike faster and you retain more control.  When you need to brake hard you have three choices: use your front brake, rear brake or a combination of the two.  Locking the rear brake will cause you to skid and lose control, locking the front brake will flip you over, but a combination of the two can stop you safely and quickly.  The ratio should be about 70% front brake, 30% rear brake.  If you don't know what the three types of brakes used in mountain biking are take a look.

But if I use my front brake won't I flip over?

Depends.  If you shift your weight back and push against your handlebars while you brake and don't lock your front brake it is highly unlike you will go over.  But if you don't know what your doing and are using disk brakes (especially hydraulic disk brakes) for the first time BE CAREFUL.  To use your front brake squeeze the lever slowly and gently.  Do not grab it and compress the lever 100%. 

Why do I stop faster using the front brake, and why do I have more control?

You will have more control braking with your front brake because your rear wheel hasn't locked up and skidded out from under you, and why are you stopping faster?  If a friend were pulling you while you were standing sideways to him and you were dragging your rear leg while a roller skate was on your other foot he would have no problem.  No switch and your front leg is resisting and your back leg is dragging, and it will be very hard for him to push you around.