How do Sophie and Teabing manage to save the keystone when Silas attacks them in Teabing's home?

When Silas attacks, in Teabing's home, Silas demands that Sophie and Teabing hand over the keystone. Teabing agrees, and seeing that he is on crutches, Silas orders him to hand over the keystone. While Teabing tries to balance the heavy marble keystone in one hand and balance on his crutch with the other hand, he loses his balance, and the keystone flies through the air. Teabing's crutch comes down on Silas with a crushing blow to his cilice, doubling the albino in pain. Sophie sees her opportunity and lands a kick to Silas' jaw. He discharges the gun, but hits no one.