Sometimes you may do poorly in the first part of a race, but feel that you have improved in the second half. The Sync feature lets you synchronise all boats at the beginning of the current leg to simulate then all rounding the mark at exactly the same time. This lets you eliminate previous performance in the race from your analysis of the leg.
To use the Sync feature, move the timeline to the desired leg of the race.
Click "Sync". 
The position of all boats will change to simulate them rounding the mark at the same time.
An orange boarder will be displayed around the map to remind you that you are viewing a simulation.