optimal play: 18.Nfxe5??
Something like 18.g3 maintains Browne's advantage.
Instead he played an unsound sacrifice and embarked upon a faulty plan which fizzled out, leaving him behind in material, and ultimately suffering the ignominy of being mated.
This game is from the 20th Chess Olympiad held in Skopje (then in Yugoslavia) 1972, played on 9th October, in the C-Final, 12th round, board 1, between Australia (white) and Finland (black).
Much was made of Browne suffering the shock loss to Abdul-Razzaq Ahmed Taha in the 3rd round of the preliminary group, but this loss to Westerinen, his only other during this Olympiad, probably cost him the individual gold medal.
Browne had to settle for the top board individual bronze medal behind Hübner (Gold) & Hort (Silver), despite the fact that he finished the Olympiad with a score of 17½/22 [+15/=5/-2] compared with Hübner on 15/18 [+12/=6/-0] and Hort 14½/18 [+11/=7/-0]
Browne's score of 17½ was the highest for that Olympiad, not just on board 1 but in fact on all boards!