Critical capture

In Generation V, there is a chance of a critical capture, where the Poké Ball will only shake on the ground once before capture. When a critical capture occurs, the Poké Ball will make a high-pitched whistling sound as it is thrown, then pause in mid-air, and shudder before it drops down to the ground.

The probability of a critical capture depends of the number of Pokémon that have previously been caught in the Pokédex. First, compute the value of a (the modified catch rate), as above. Then, whether or not this value is less than 255 (above which the Pokémon is caught regardless of whether or not it is a critical capture), multiply this value by the following:

Species Caught Multiplier

>600 2.5

451-600 2

301-450 1.5

151-300 1

31-150 0.5

<30 0

A random number is generated, and if it is less than the modified value, a critical capture occurs.

Critical captures make one shake check instead of three, thus it appears to increase the odds of capture for Pokémon. Though it may create higher odds of capture, a critical capture is not a guaranteed catch; it is possible for critical captures to fail. For example, a critical capture against a full-health Pokémon with a catch rate of 3 (such as Kyurem) with a Poké Ball will still only have a 25.03% chance of success, but it is much higher compared to the normal capture's 1.57% chance. Furthermore, situations where critical capture is guaranteed will not guarantee capture.

///source bulbapedia