How it is calculated
It is the Pokémon's usage among Top 8 teams divided by its usage across all teams. If a Pokémon is on 20% of teams and on 26% of Top 8 teams, its index is 26 / 20 = 1.30.
A 1.00 means it reaches the Top 8 in the same proportion as it is played. Above that, it outperforms its usage; below, it underperforms.
How to use it
Look for Pokémon with an index above 1.2 and a decent sample: they are the ones that show up on winning teams without being what everyone already brings. They are often what the meta ends up copying a week or two later.
A very low index on a very popular Pokémon also tells you something: it is played a lot but falls short, perhaps because everyone already knows how to stop it.
Its limits
There are only eight teams in each Top 8, so with few tournaments the index jumps around: always read it with its number of teams next to it. And like win rate, it also measures players: the best players pick the Pokémon they think are strongest, which pushes the index up.
On infovgc you will find it on the meta page and on every Pokémon's page, next to usage and adjusted win rate.
