Hi
If you want to start monitoring the markets (refreshing the prices) 10 mins before the start of each race, use the following setting:
Settings > Monitoring Options > Market Refresh > Start monitoring events at 10.00 mins before the beginning
If you want the trigger to fire 10 minutes before the off, use this condition in the betting trigger:
and markets minutes before the off is equal or less than 10.00
Of course the trigger will not fire if the market is not being monitored.