The AC8340 is a gas spring-equipped monitor bracket and is configured for a certain weight of the monitor and can be adjusted to fit the weight of your monitor.
Locate the Allen bolt shown below
Keep the monitor in the middle by lifting it up or pushing it down.
Now use the Allen key to adjust the tension of the gas spring.
If the monitor lowers when released, turn the Allen key to the + and repeat until the monitor stops lowering.
If the monitor rises when released, turn the Allen key to the – repeat until the monitor stops rising.