The best way to do this is to use the new beta software, see the thread in the Software Dev forum.
Set the record length to whatever you want in Record Settings->Record Modes, then set the trigger delay to 0% in Record Settings->Trigger Delay. Turn off Invert and Pullups in Trigger/IO settings, then start recording and leave the camera recording. Once your TTL event occurs, it will save from the point the TTL signal went high, for the duration specified in Record Modes.
I believe the camera may have been capturing only one frame because it was triggering immediately when you started recording, this depends on your settings however.