Their bodies copy the twins: their human behavior, flexibility and acrobatic skills.
The right half of their brains contains affection for fire, strategic thinking and half of their memory.
The left half contains submission to the Witches, silent but violent behavior and another half of the memory. It basically makes them the obedient destructive zombies that they are.
Brainless scarecrows have neither the pyromaniacal tendencies, nor the mindless obedience to the Witches, and show actual individuality and personalities.
They have to be treated with one of Pepper’s potions that helps them last longer; otherwise the materials they’re made of spoil rather quickly.