A loud freehub and shouting “passing on your left” rarely does the job you intend it to. Timber! is different. It’s a bell, but not as you know it.
This original bicycle bell is intended for mountain biking. A lever switches it ‘ON’, releasing the clapper and allowing it to sound with the motion of the trail. Raise the lever to switch it ‘OFF’ when not needed and silence is restored.
The familiar sound of a bell, alerting other trail users before you see them. It’s the responsible way to ride shared-access trails.