Ok, I remember that one (late 80's), but I never used the Porta Two.
The sync function would be used when you want to record a sync tone from a drum machine or sequencer to track 4 of the recorder. You can then keep the drum machine/sequencer in step with the recorder by feeding the recorded sync signal into the drum machine or sequencer sync in. You lose a track for the sync tone, but you get drums and sequencers in exchange.
Is there a switch or jack labeled “Sync” on the Porta two? If there's a switch it just disables DBX on track 4. Dbx interferes with FSK tones used for tape sync.
I've always just put my tone on track one, then used the tape cue out to send it back to my sequencer.
I guess maybe the dbx is bypassed through the tape... I don't know, but I've never had a problem with the dbx causing it to not lock up. I always kept four open in case I wanted to bounce you know?
It makes sense if you have the tape cues dedicated though.