Very simple. Someone names an anime character, and the next person names who they think is the perfect voice actor for that character (in other words, any voice actor whatsoever). Here's a few examples.

Poster #1: Kiyone Kotetsu from Bleach?
Poster #2: Mona Marshall. Prussia from Hetalia?
Poster #3: Rik Thomas. Megatron from Transformers Armada?
Poster #4: Steve Blum. Ken the Eagle from Gatchaman?