Perhaps Ezra was on his knees and thus from a kneeling position he could repeatedly raise himself and then throw himself down again. More likely, however, he was praying on his knees with his arms outstretched (Ezra 9:5), and then he fell face down, but only once.
1 While Ezra prayed and made confession, weeping and casting himself down before the house of God, a very great assembly of men, women, and children, gathered to him out of Israel, for the people wept bitterly.