This demonstration is being done on a shoulder holster, but it can easily be adapted to work with any sidearm holster that uses a snap type thumb break.  Most tactical holsters you come across will have a snap type thumb break.  Some are not adjustable so this will not apply to them, but most are.  My Blackhawk Omega VI Assault Holster has an adjustable thumb break, so this tells me that the higher end holsters do as well.

Not only will I show you how to index the thumb break, but I will show you how to draw your weapon quickly and efficiently.  This is shown for a right handed person, if you are left handed, switch sides.

Step 1: The first thing that you want to do is to set the holster up in a position where it is comfortable and convenient to acquire your sidearm.

Step 2: Holster your weapon in the fashion in which it will be under normal rest.

Step 3: Place the female end of the thumb break along the side of the gun.  This will allow you to 'wrap' the male end of the thumb break over the end of the gun.  Here are some different looks and angles.

Step 4: By setting up your thumb break in this fashion, it allows your thumb to naturally end up on the inside of the snap.

Step 5: As you continue to 'grab' your weapon, make sure that your thumb is firmly pushing on the break.  With a rigid hand, the thumb break will snap open very quickly and easily with little to no effort.

Step 6: At the same time you are opening the thumb break, your hand should be positioning itself into the correct tactical grip on the gun.  REMEMBER, finger off of the trigger, you do not want to end up shooting 'yourself' as you draw the weapon.  Basically it will look similar to this.

Step 7: After the thumb break has being disengaged, you will then lower your thumb onto the weapon to complete your grip on the weapon.

Step 8: You will then be able to pull the weapon from the holster and you should already have a controlling grip on the firearm.

Step 9: You are now ready to engage.  Here is a small sequence of how it would go.