Complex sentence: Mansell was leading the race on lap 184 as the field was coming to a restart, but his inexperience on oval circuits caused him to misjudge the restart speed and he was quickly passed down the mainstretch by Fittipaldi, which proved to be the winning move.
Split into simpler sentences: Mansell was leading the race on lap 184 as the field was coming to a restart. His inexperience on oval circuits, however, caused him to misjudge the restart speed, and he was quickly passed down the mainstretch by Fittipaldi, which proved to be the winning move.