If idle time charge is encountered before reaching minimum mileage, the equivalent amount of distance (by price) is taken from the min mileage. You'll see it being deducted in the min mileage indicator (shown as a negative distance left).
When min mileage is reached, idle time charge is added to the meter instantly as it happens.
Other options:
Add to meter instantly. Regardless reaching minimum mileage or not, charge for the idle time will be added to a meter immediately.
Throw away. Don't charge idle time until minimum mileage is fulfilled.
Keep for later. Keep the idle time charge hidden until min mileage is reached. On reaching it add the "saved for later" charge. Causes meter to "jump" when min mileage is reached.