Details for this are now known! Here is a simple summary:
The probability of a critical capture happening is similar to the probability that you will capture that Pokemon. So if you had a 30% chance of catching a Pokemon with a Pokeball, when you throw it there is first a 30% chance of a critical capture. If the CC doesn't happen then it has a 30% chance of catching the Pokemon as normal.
However, the chances of a CC also increase the more Pokemon you have in the Pokedex (see below). So if you had over 600 Pokemon in the Pokedex, using the above situation you would now have a 75% chance (30x2.5) for a CC to occur. Again, if the CC doesn't happen then it has a 30% chance of catching the Pokemon as normal.
If a critical capture does happen, the chances of capture do increase significantly but it's not guaranteed. I'm not 100% clear exactly how much the chances increase.