How does a fast ball work?

Question

The description says it works best on Pokemon that are fast or quick to flee, but what does this mean?
Roaming Pokemon?
Pokemon with high speed?
Or is it like a quick ball in that it works best on the first turn?

Answer 1

In Gen 2, it is supposed to be 4x more effective on Pokémon with a chance to flee, but due to a glitch, only applies to Tangela, Magnemite, and Grimer (all of which have a 10% chance to flee every turn).

In Gens 4+, it has a 4x increased catch rate on any Pokémon with a base speed of 100 or more, except Ultra Beasts, which always have a 0.1x rate.

Source

Comments

Cool, Thanks!

Answer 2

The fast ball makes Pokemon that have a high speed easy to catch, but it also allows you to catch Pokemon like abra that may run / teleport. This ball is useful when catching electric types since they have quite a high speed stat.