How does Asterix use his cunning to rescue Getafix?

Asterix fools the guards into thinking he is strong so he can gain access to Getaflix. Then the two plot together against Crismus. Asterix feigns fear so that Crismus orders Getaflix to make the potion for release of his friend. Getaflix, however, makes a different kind of potion and Asterix pretends to get hurt in a mock demonstration, thus securing their release.