How do you down- shift a Volvo 10 speed transmission tractor trailer?

When your RPMs get down to 1000, you take it out of gear, press the accelerator to tach the motor up to 1500 RPMs, and drop down into the next lowest gear. Assuming you're operating this truck in North America, your transmission will either be an Eaton-Fuller or Spicer 10 speed unsynchronised transmission, and you must tach up to motor to compensate for the lack of a synchroniser gear in the transmission.

If you drop your RPMs down to 700 - 800 before downshifting, you can skip a gear in downshifting, and downshift two gears.