Howard: Chess Life analyzed this game back in late 1980, as I recall. Still remember playing it over on my pocket chess set while in my college dorm room one evening.
Regarding the tiebreak rules, they went as follows....if after ten games the score was tied, then the two players would play a two-game tiebreaker (regular time controls). If that didn't settle it, an additional two games would be played.
That would therefore make 14 games in total, and if the score was still tied then whoever had won the most games with Black would be declared the winner. If that wasn't enough to determine a winner, then the last player to have won a game would be the winner.
Rather strange way to break a tie, but then Spassky and Portisch had agreed to all this before the match started--thus, it was fair.
As an earlier kibitzer mentioned, Portisch won Game 1 (this game) and he'd had Black. Thus, he was already at an advantage right from the get-go ! Spassky won the ninth game, but.....he'd had White. The other 12 games were drawn, and Spassky thus had to go home.