Miter saws ship from the factory with the saw arm locked down. It's secured in the down position. When you first use the saw you will need to release the arm and install a blade if one isn't already installed. The saw arm should raise to the up position by itself when you have unlocked it. If it doesn't, it means the travel pivot may need adjusting. The saw can be repaired at a service center if this occurs.
Push down on the "D" handle and cut the tie-wrap. Keep pressure down on the "D" handle while you do this so that the arm doesn't spring upwards.
Pull out the lock pin by rotating it 90 degrees. The saw blade arm will now be free.
Release some pressure on the "D" arm, and the arm and installed blade will rise.
To lock the saw arm, firmly grasp the “D” handle and push down with steady pressure. At the same time pull the lock pin out and away from the housing. Then just release the lock pin allowing it to lock the saw.
Always make sure the saw blade is clear of all obstructions before turning the saw on.
Wait for the saw blade to stop turning before raising the blade.