How does Asterix fool Crismus Bonus into thinking that he and Getaflix are at Crismus' mercy?

Asterix tells the Centurion that he does not know the recipe for the potion and Crismus Bonus calls the torturer to convince Getafix to reveal the secret to save his friend. As the torturer ties Asterix to the table, Asterix cries out for mercy, and Getafix agrees to make the potion for the Romans.