From Bulbapedia, the community-driven Pokémon encyclopedia.
Five players competing in 'Berry Crush'
Berry Crush is a minigame in Pokémon FireRed, LeafGreen, and Emerald. It is similar in function to the Berry Blender minigame found in Ruby, Sapphire, and Emerald. It is played in the Union Room and can have up to five participants at one time.
To use Berry Crush the player must repeatedly press the 'A' button as fast as possible. Though similar to the Berry Blender, this game makes Berry Powder rather than Pokéblocks (as these have no use in the Kanto region).
After the Berries have been crushed, the player will get some powder. This can be traded for certain items. The powder is stored in the Powder Jar, which can be obtained from an old man in a house in Cerulean CityFRLG or from a woman in the Slateport City outdoor market E. These characters will also give away certain items depending on the amount of powder that the player has.