Answer:
If the driver blinks his lights after you pass, it's a signal that it is clear to pull back in. Be sure to move back only when you can see the front of the truck in your rear-view mirror. After you pass a truck, maintain your speed.
Step-by-step explanation:
Truck drivers use their lights to communicate with other drivers. If an approaching truck flashes you, it usually means danger ahead.
While you are overtaking a truck, a blink in the truck's lights means that you can safely pull in since there is a safe distance between you and the truck. Without the blink, you would have to judge the pulling in clearance yourself, which may be difficult sometimes, especially with very long, articulated trucks.
Flashing any kind of vehicle lights is prohibited in many countries.