Modelling Light
A continuous lamp built into a flash head that previews where the shadows will fall.
Measured in
W
In depth
Because flash is invisible in use, the modelling light is the only way to judge shadow placement and specular position before firing. Setting it to track the flash power proportionally lets the relative brightness of several heads be compared by eye. It is far too weak to contribute to the exposure at typical flash apertures.
Examples in practice
Switching off all but one modelling light shows exactly where that head's shadow falls across the subject's face.