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The instructions on this page will guide you in modifying a fire simulation so that it emits only when the emission object moves. This can be useful for effects that occur with animation and leave a trail behind, such as smoke trails from a vehicle or sparks flying from a magic wand.

Start by creating a new project from File → Project Window. 
Click the New button at the top right corner of the window and give it a name.

This is important because Phoenix stores the simulation data in the data directory of the currently selected project. You can manually point to a different project directory from the File → Set Project... dialog.

Create a piece of geometry to use as an emitter. In this example, a default poly sphere is used. 

Rename the geometry to source_geo_01. 

With the source geometry selected, click the Fire preset button on the Phoenix Shelf. 

This will automatically generate a fire setup that you can play with. 

Start the Simulation. 

Note that no fire is produced. This is because our object is stationary and the modifier is multiplying the discharge by 0. 

Increase the Value for Position 0 on the Discharge modifier's ramp to 1, and for Position 0.01, set it to 0.

This means that whenever the object is at rest, emission will occur but the moment the object starts moving, the Discharge will be 0.

Add some animation to your object and Start the simulation to test this. 

At the moment, the object is emitting fire only while stationary.

To reverse this effect (i.e. to have it emit fire only when moving), you need to reverse the Discharge diagram.

The X-axis of the diagram is the input values, the Y-axis is the result. Therefore, for input values of 0 (i.e. speed is 0, object is stationary), the Y-axis value should also be zero. When that condition is met, the object won't emit when its standing still because its speed is 0.

You can see this setup on the image to the right.