One of the most memorable events of the original Pokémon anime is "Who's That Pokémon?" These segments would typically start immediately before each commercial break by providing a silhouetted version of a Pokémon, which would be revealed immediately following the commercial break. This was not only a fun way for viewers to interact with the show, but it also helped the show retain its viewership through commercials.
Source：https://www.cbr.com/hardest-whos-that-pokemon-anime/
