How do Getaflix and Asterix fool the Romans with their potion?

Getaflix says that he can make the potion without the strawberries and it just will not taste as good. The potion is made, but no one is willing to try it. They give it to the cart seller and look for a volunteer to be hit to see if it works. The guards evade Crismus Bonus' request by conversing in Latin. Asterix volunteers and falls to the ground as the cart seller hits him.