Jace just had enough time to steady himself before Sebastian was on him. Sebastian pushed Jace to the ground and tried to choke him by pushing all his weight down on Jace's throat.
Jace was just about to give up when he thought of Clary. She was probably screaming for him. He couldn't let this monster have her, she was his and he was hers. With all his strength Jace pushed Sebastian off him and leapt to his feet.