The Doctor saw Jack go down and the fury blazed through him again. Somewhat fortunately, it froze him - blanching sharply - into place rather than letting him give in to the impulse of that first frantic heartbeat, which was to fly at the Master and try to throttle him with his bare hands.
Instead, he stood helplessly staring at the Master for endless seconds, before gritting his teeth and looking down to fumble with his sonic screwdriver.