The Master saw his alternate line up his shot, gathered up Romana and shoved her hard out of the way. He wasn't letting Romana take the hit if his life depended on it.
It did. He stumbled into the line of fire, and went down, his hearts seizing from the blast. The sheer will to live was holding for above consciousness, but he could feel his body failing.
Romana fell, scrambling to back back to her feet and be useful.
The Doctor had lined up a shot, but missed as the other Master ducked away. He flung himself at his partner. "No, no, no, no." out his mouth, before he could really process what was happening.